One summer day, nine-year-old Keira Ball was in a terrible car accident and suffered catastrophic brain injuries. As the rest of her body began to shut down, her heart continued to beat. In an act of extraordinary generosity, Keira’s parents and siblings immediately agreed that she would have wanted to be an organ donor.

Meanwhile, nine-year-old Max Johnson had been in a hospital for nearly a year, valiantly fighting the virus that was causing his young heart to fail. When Max’s parents received the call they had been hoping for, they knew it came at a terrible cost to another family—in what Clarke calls “the brutal arithmetic of transplant surgery.”

The act of Keira’s heart resuming its rhythm inside Max’s body was a medical miracle. But this was only part of the story. While waiting on the transplant list, Max had become the hopeful face of a campaign to change the UK’s laws around organ donation.

Following his successful surgery, Keira’s mother saw the little boy beaming on the front page of the newspaper and knew it was the same boy whose parents had recently sent her an anonymous letter overflowing with gratitude for her daughter’s heart. The two mothers began to exchange messages and eventually decided to meet.