Ollie and Moritz are best friends, but they can never meet. Ollie has a life-threatening allergy to electricity, and Moritz's weak heart requires a pacemaker. If they ever did meet, they could both die. Living as recluses from society, the boys develop a fierce bond through letters that become a lifeline during dark times--as Ollie loses his only friend, Liz, to the normalcy of high school and Moritz deals with a bully set on destroying him. But when Moritz reveals the key to their shared, sinister past that began years ago in a mysterious German laboratory, their friendship faces a test neither one of them expected.