This is the tale of a boy, a girl, and the heart they share: Max and Keira.

Max had been a footballing, tree-climbing, play-fighting force of nature until a mysterious illness caused his heart to fail, leaving it so dangerously weak and unstable he was forced to spend nine months confined to a hospital bed.

Max’s heart muscle had been fatally harmed, probably by a mild viral infection he had scarcely noticed. While he was in hospital, his parents acquired the new and terrible knowledge that up to one in five children in Britain and the US might die while waiting on the list for a heart transplant. They were equally aware that the only thing that could give Max what he needed to live was the death, appallingly, of someone else’s child.

Keira was nine, a child alight with the act of living, so besotted by animals she could not walk past an upturned snail without having to stop and right it. She loved looking after her horse even more than she loved riding it, dreamed of one day working with animals, until she had the terrible misfortune to suffer a catastrophic brain injury as a result of a road traffic collision. Keira’s family, upon being told she was brain dead, immediately decided to gift her organs. They knew with absolute certainty that this was what their daughter would have wanted.

So, from the moment Keira was fatally injured, her heart began a journey so momentous it was scarcely believable. First, there were the emergency chest compressions at the scene of the crash – a junior doctor’s palms bearing down with all their might, striving to keep oxygen flowing through her body. Next, the strange metaphysical limbo between life and death as Keira lay in intensive care, warm, flushed, apparently sleeping, yet somehow – unfathomably – brain dead. Then the moment when her heart was stilled by an anaesthetist’s drugs so that the surgeons, silently at work within the cave of her chest, no longer faced a moving target. From there, the light-aircraft dash halfway across the country to deliver the organ, chilled on ice, into gloved and poised surgical hands. Finally, the intricate knitting of the heart’s great vessels into another child’s torso – and the agonising wait to see if its chambers would resume their vital work.

It was seven months before Keira’s mother Loanna Ball’s car collided with a pickup truck in Devon that the lives of Max’s parents, Emma and Paul Johnson, were overturned. On 9 December 2016 Emma found herself sitting in the relatives’ room of the paediatric intensive care unit (PICU) at the Royal Manchester Children’s Hospital opposite one of the hospital’s most experienced paediatric cardiologists, Dr Salem Rahouma – tall, imposing, Libyan by birth. Between them, strategically placed, was a box of NHS tissues. A few doors away lay eight-year-old Max, barely conscious in an intensive care bed.

From time to time – too often in Salem’s specialty – doctors are required to utter words that drop like stones upon a parent’s chest, crushing air and hope from the lungs. Salem set about explaining, with the help of an NHS paper towel – the only writing surface at hand – how Max’s heart was enlarged by a condition called dilated cardiomyopathy. He drew the four chambers, the valves, the aorta, vena cava and pulmonary veins and arteries, explaining how the heart had become so swollen with blood that it was too weak to pump away. The problem with Max’s heart, Salem said, is a condition that has a 33% chance of resolving, a 33% chance of needing a heart transplant, and a 33% chance of death.

Max’s illness had emerged so insidiously that Emma and Paul scarcely noticed at first. A dry cough, mild and persistent. By early autumn, the cough was accompanied by wheezing and moments where Max seemed to gulp for air. A visit to the GP resulted in a diagnosis of asthma and a bag of inhalers, none of which seemed to help. By November, he was too tired to sprint across the football pitch at Scouts. Back at the GP, Emma’s concerns were finally heard. Auscultating Max’s chest with a stethoscope, the GP found a heart murmur and agreed to refer him for a chest X-ray and cardiology appointment. When the X-ray appointment came through a few days later, he could barely walk into the hospital. Within days, Max was admitted to Manchester’s children’s hospital.

For a while the function of his heart was stabilised with an intravenous drug called milrinone. “It’s a bit of a miracle drug,” says Clair Noctor, the paediatric specialist nurse who met Max on the first day he arrived on Ward 85. “Max perked up on it. He was in really good spirits by Christmas, animated, very chatty.” For all the superficial improvement, Clair was under no illusions that Max, in all likelihood, was going to require a heart transplant.

Nor was Salem Rahouma: “I remember the severity of Max’s condition. It is rare to see a cardiomyopathy so severe. I knew he could die at any time.”

From Christmas Day onwards, the decline was precipitous. Increasingly frequently, Max became quiet and still, lying on his side in order to breathe. His waterlogged lungs, bloated with the fluid that his heart could no longer pump away, sometimes caused him to feel so nauseous he would end up vomiting. “You could see his breathing become shallow and fast,” says Paul, “and he’d become so breathless he’d struggle to get enough oxygen into his system and start ‘gulping’ for air. It was awful to watch.” One day, Paul noticed a bump in the middle of Max’s chest, where his sternum had begun to protrude outwards. When the doctors explained that the deformity was being caused by how grossly overstretched Max’s heart had become, Paul grew silent.

By now, Max’s team was in daily contact with the paediatric cardiologists at the Freeman hospital in Newcastle, hoping he could be imminently transferred there. The hospital performed the UK’s first successful heart transplant in a baby in 1987.

With years of experience talking to families about heart transplants, Clair introduced the topic to Emma and Paul “almost by osmosis”, recalls Paul. “It dawned on us gradually that Max’s heart was beyond repair and no longer fit for purpose. Max was going to need a new heart.”

By the time Joe Ball’s motorbike screeches into Bristol on the afternoon of Sunday 30 July 2017, his family, in every sense, has been shattered. Joe had been on a road trip with friends when his wife, Loanna, daughter Keira and son Bradley were gravely injured in a collision on the A361 near their home in Devon, a route with the dubious accolade of being one of the most dangerous rural roads in Britain. To the north of the city, in Southmead hospital, Loanna is barely conscious in intensive care, with multiple fractures of her arm, hand, foot and ankle. Further south, in the Bristol Royal Hospital for Children, seven-year-old Bradley is still losing blood from a ruptured spleen and needs immediate surgery. Back in the family’s hometown of Barnstaple, a surgical team at North Devon District Hospital is struggling to keep nine-year-old Keira alive.

Joe is being wrenched in three different directions at once. He knows – indeed, it is the only thing that makes a shred of sense to him – that his family cannot remain splintered. For Loanna, this is easily achieved. She can be transferred across the city by ambulance to the Bristol Royal Infirmary, adjacent to the children’s hospital, where Bradley is currently undergoing emergency surgery. But for Keira, the three-hour ambulance journey from Barnstaple could be fatal. To the legitimate concerns that Keira could die in transit, there is one irrefutable rejoinder: what if Keira dies alone in a hospital a hundred miles from either parent? In what sense could that be better?

Conversations fly back and forth between the intensive care teams in Barnstaple and Bristol. Finally, at 10 o’clock that night, a PICU retrieval team safely delivers Keira by ambulance, on a ventilator, to Bristol.

Even before she arrives in the children’s hospital, the PICU team explain to Joe that his daughter’s injuries are so catastrophic she is unlikely to survive.

The most important scan of the night, a CT angiogram of Keira’s brain, takes place at 2.31am precisely, on Monday 31 July. Already the PICU team fears the worst. They know that Keira’s level of consciousness on the Glasgow Coma Scale (GCS) registers only a 3. A fully conscious individual effortlessly scores the full GCS 15, whereas, at the opposite end of the scale, Keira’s GCS of 3 means she makes no response whatsoever to voice, touch or pain. Everything points to Keira being brain dead.

Later that morning, Keira’s sisters, Keely and Katelyn, have rushed from Devon to be by her side. Desperate to see her, they clamber into her bed. With Symphony by Clean Bandit blasting out on repeat, they chatter about hair, nails, horses, music, exactly as though it is a conversation all three of them are sharing. They soften Keira’s lips with strawberry lip balm and gently elevate her hands, painting each nail a dazzling shade of orange.

It would be easy to assume from their laughter and exuberance that they are simply too young to understand the gravity of her injuries. But that would be to do them a disservice. For their efforts to treat Keira as a living, loving, conscious sister stem not from naivety but from its opposite. Their eyes dart as they monitor the nurses’ every word and gesture. They listen, grasp and absorb it all, the bleak and unforgiving facts of the matter.

As day wears into evening, Dr Sarah Goodwin, the consultant responsible for the unit that night, begins her shift. Nothing about brain stem testing or the concept of brain death – let alone the idea of organ donation – has yet been broached. “I was really just going in to say ‘Hi,’” Sarah recalls. But what happens next floors her. In response to being asked if anyone in the family has any questions, Katelyn, who looks almost identical to Keira, turns to the doctor. “She looked directly at me and said, ‘Can we donate her organs?’” says Sarah. “I looked straight back at her, because I was not expecting that at all, and then I said, ‘Yes, absolutely. We will start looking into that for you, if that’s what you would like us to do.’ Joe thought about it and said, ‘Of course she’d want to be an organ donor. Keira is the kind of girl who gives her last sweet to her sisters.’”

At 2.46am on 1 August Sarah picked up the phone and called the UK’s national Organ Donation and Transplantation Hub, the nerve centre through which every transplant is coordinated across the whole of the UK. She informed the coordinator on the end of the line that there may be a prospective paediatric donor in Bristol.

Tucked behind a large Sainsbury’s supermarket and a nondescript retail park just off the Bristol ring road is a low, squat building that most people scarcely notice. The corridors within are equally drab. One hallway extends into an open-plan office containing 12 desks, each with a worn-out computer on top. In these decidedly humdrum surroundings, at exactly 10 minutes to midnight, a chain of events begins with the potential to end in wonder. A telephone rings. The on-call specialist nurse in organ donation at the Bristol Royal Hospital for Children is on the line. Having completed an exhaustive round of information-gathering, investigating and double-checking, she is calling formally to register nine-year-old Keira Ball as a donor whose organs are ready to be listed and matched.

A coordinator enters Keira’s age, height, weight, blood group and tissue type into the national donor database. A second coordinator cross-checks the data independently: human error in recording these vital facts could be calamitous. Next, the coordinator assigns an ID number to Keira to ensure her anonymity. With one click of a mouse, the algorithm fires, and after two or three minutes of permutations, a ranked list of matches beings to fill the monitor. Matters of life and death, in spreadsheet form.

There are 13 children in need of urgent or super-urgent heart transplants who match Keira’s size and blood group. The child at the top of the list is the same age as Keira, nine years old. His name is Max Johnson.

By this time Max had been on the transplant list for 196 days, having moved a few days after his ninth birthday to the Freeman in Newcastle, where a titanium pump had been stitched into his heart to keep it beating. The longer he languished in bed, too weak and breathless to walk, the more depressed he became. “I was just at the point where I thought, you know, I’ll be stuck in here for ever until I die,” he says. “It’s like I was waiting to go. I was getting really frustrated, like I was jealous of other kids. Because you could see the park across the way where kids were playing football. I just wanted to be doing that too.”

Paul and Emma’s lives were on hold too: unrelenting stress with no end in sight. “Waiting on the transplant list had begun to feel like being on death row. It was torture, purgatory, highly emotional, and both physically and mentally draining,” says Paul. As soon as Emma was told a match had been found, she called Paul at home in Cheshire. “Emma said, ‘He’s got one, he’s got one, you need to get here straight away.’ I just remember it was like a dam bursting.”

By 1.50pm on 2 August in the operating theatre at the Bristol Children’s Hospital, the surgical team has assembled for the retrieval of Keira’s organs. They pause for a moment of honour: a time of reflection and appreciation of the selfless act of kindness and generosity from the donor and their family. In his surgical headlamp, Pradeep Kaul, the cardiothoracic surgeon who will remove Keira’s heart, looks like a miner readying himself to dig, to bring to the surface buried treasure. A heart, a liver and two kidneys [which went to other fatally ill young patients]. You cannot put a price on four lives saved. A scrub nurse places a scalpel in an outstretched hand. Knife to skin. A ribbon of crimson.

By 3.12pm Keira’s organs can be removed from her body. The order in which they are retrieved is determined by how long each of them can tolerate being deprived of oxygen. Studies show that excised hearts can endure cold ischaemia – being embedded in ice to maintain a core temperature of approximately 4C – for at most six hours in young children. Livers, on the other hand, can be cocooned in ice for up to 12 hours without survival rates being affected. For kidneys, it is up to 24 hours.

Within 12 minutes Keira’s heart has been sealed inside a cool box filled to the brim with crushed ice. Two minutes after that, it has left the building, whisked straight into an ambulance, and then to Bristol airport where a private plane will fly it to Newcastle, ready to be collected and rushed straight to the operating theatre where Max will be lying, draped and unconscious, his own chest splayed.

It is not called a theatre for nothing. Spotlights converged from above on cardiothoracic surgeon Asif Hasan, as his performance approached its dénouement. After the slog of the dissection, he could not resist a moment of playing to the crowd. He tugged Max’s heart out of his chest, titanium pump still attached, and glanced briefly at the shattered tissue before dumping it unceremoniously in a plastic bowl. “Throw it in the bin,” he growled dismissively.

A humming, whirring mechanical oxygenator now performed the role of Max’s heart and lungs. The stage, at last, was set for Keira. Only she could resurrect this boy whose life was all but over. A theatre nurse prised the lid from the cooler box and swept away a coat of ice chips and plunged her gloved hands into crushed ice. Triple-bagged, submerged in fluid, a perfect yet petrified living organ.

It is time. Asif lifts Keira’s heart as though it might bolt from his grasp if startled. Every eye is glued to the hands that clasp the promise of life for the boy on the table. This isn’t medicine, it’s witchcraft. Asif places the heart in the empty cavern of the opened chest. His wrists disappear inside Max’s body and, together with fellow surgeon Fabrizio de Rita, they begin to stitch. Two adults and two children are briefly one, four people sharing the same shell of a body. Slowly, methodically, the surgeons suture severed aorta to severed aorta, severed vena cava to severed vena cava. Each great vessel is married to its mirror part, Keira to Max, Max to Keira. It is the neatest, most uniform, most precise needlework. And while they sew, the surgeons know, a secret alchemy is under way. Keira’s heart is becoming warmer. Dozily, dreamily, her myocardial cells are waking up, becoming a little more metabolically active. More than anything, this heart aches to beat. Honed by millennia of evolution, it exists with one aim alone, a single-minded thirst for pumping. There is a quiver, a shudder, a microscopic gyration. The quivers – fibrillations – now swiftly resolve into recognisable contractions. Keira’s heart, now Max’s heart, is moving, beating, in an unfamiliar chest, tethered to unfamiliar vessels.

Finally, they ask the perfusionist to gradually dial down the support Max receives from the heart-lung machine. All eyes on the heart, nobody breathing. It is pumping now without any external assistance and Max’s blood pressure seems to be stable. Five, 10, 15 minutes elapse. Still Asif waits, watches. Still the blood pressure holds. Finally, he nods at Fabrizio to release the retractor and slowly, tectonically, the battered ribs begin to ease back into place, shrouding once more the crypt of Max’s chest, the vault in which Keira’s heart has been laid to rest.

The day after Max’s transplant Emma cannot stop looking at his cheeks. “They were pink, he was pink. I’d not seen him like that for nearly a year.” Within three or four days, Max’s cheeks are plump as well as flushed, since he can finally process food again. His vision, which had been blurred, rapidly improves once his retinas are no longer starved of oxygen. A week after the transplant, the nurses don’t know whether to cheer or restrain him when suddenly he sets off at a run down a corridor.

Either Paul or Emma arrives on the ward each morning to prepare and administer Max’s immunosuppressants. Without these drugs his body could reject his new heart at any time. In order to build up their confidence in supervising his medication at home, Emma and Paul lay out the drugs on the hospital tray themselves – cyclosporin, prednisolone, azathioprine, aciclovir. For the rest of his life, Max will need to take precisely timed, twice-daily doses of cyclosporin, or an equivalent drug, to prevent his body rejecting his heart.

On 14 September, just six weeks after his operation, he is told he is well enough to go home. Shortly afterwards, Emma sits down to write a letter to Keira’s parents. “We wanted to capture our feeling of gratitude, but also recognise their loss,” says Emma. “I just tried to write from the heart by imagining myself in the shoes of the person who had lost a child.”

As per the strict rules governing contact between donor and recipient families after a transplant, Paul and Emma are permitted to use only their first names in their letter, which is sent to Joe and Loanna via the intermediary of NHS Blood and Transplant. Ordinarily, these precautions protect families from the potentially traumatic experience of discovering each other’s identities. In Max’s case, however, a detailed account of his story has already appeared on the front pages of a national newspaper. Even before Keira’s family receives Paul and Emma’s letter, they suspected that her heart may have been given to Max, a boy the same age as she was, who received his transplant around the time that Keira’s organs were retrieved. As soon as Loanna opens the letter and sees the names Max, Emma, Paul and Harry, her theory is confirmed.

One day, nearly three months after Max’s transplant, the family are packing to go away on a short family holiday when Emma quickly checks her Facebook page. She sees she has just received a private message. It is from Loanna and it reads: “I think you may have our daughter’s heart and it’s the most beautiful heart in the world.”

The two families begin to discuss the idea of meeting. Emma and Paul are initially hesitant, concerned about causing Joe and Loanna unintended pain. In fact, both Joe and Loanna derive enormous comfort from knowing that some of Keira still exists within Max.

Nine months after Max’s transplant, Keira’s entire family travels to Cheshire to meet the Johnsons at home. The most heart-rending moment of the afternoon occurs when Emma produces a stethoscope, borrowed from a friend, in case Keira’s family would like to listen to her heart inside Max’s chest. Laughing with disbelief and wonder, the children form an excited queue to hear their sister’s heartbeat. Then it is the turn of Joe, the father who chose to surrender his daughter’s heart to honour her abundance of kindness. Staring at Max’s chest, skinny, disfigured and undulating visibly to the indefatigable beat of his daughter’s heart, Joe places the buds of the stethoscope in his ears, takes a deep breath, and listens.