After escaping from two very different prisons, Toby and Epsilon 17 finally have a chance to live for themselves. Helping to build a new city in the wake of the destruction of the Institute should be all they're worrying about, but Epsilon 17 has a horrific secret that's getting harder and harder to hide. Cassandra isn't dead, she's locked up in the deepest, darkest corner of E17's mind. Pushing E17 to the brink of madness, Cassandra is determined to take over E17 entirely and destroy the rebellion. Can Epsilon 17 overcome their hidden enemy and learn to trust the people around them? Unwilling to wait for Cassandra to force their hand, Epsilon 17 decides to take control: to go to a city where the Institute still holds sway, and try to destroy them once and for all. Toby, forbidden from joining the mission, has to find his own path forward. Their connection is as strong as ever, but the distance between them keeps growing.