As billions die from an apocalyptic cloud of polluted dust, David-a climate activist in a previous life-is holed up in an abandoned hospital in London offering a cure to anyone who needs it. Months earlier he escaped from a rogue government scientist whose illegal experimentation gave David two unasked for gifts: the ability to cure people with his blood and a locked collar around his neck. When Lysander, the sheltered son of an aristocrat, arrives and begs David to save his mother's life, David has to try. Together, the two young men begin a journey across the UK toward an island community they hope will provide food and shelter.

Lysander struggles, longing for the security he took for granted before society crashed. He despairs over David's hero complex and tendency to lead them both into danger. David is torn between protecting Lysander and the guilt that drives him to use his power to save the dying. Their clashing personalities, values, and ideas of what safety looks like, are woven together with an irresistible attraction that grows into something deeper. As they face the elements, other survivors, and a government desperately trying to reassert control, David and Lysander's greatest challenge becomes staying together while fate conspires to rip them apart.