The group faces a threat when they discover a new refugee is a spy working for Sentinel Services; Reed visits his estranged father and learns secrets that could prove useful.

Polaris learns more about her past and makes a crucial decision that could have impactful consequences. Meanwhile, the Mutant HQ comes under attack and, with everything to lose, relationships are put to the test and alliances shift.

Thunderbird contacts a mutant lawyer who recruited him to the Underground years ago, hoping she can help him find the Inner Circle. Lauren and Andy have a shared dream, which distracts Andy from his training and prompts Reeva to consider cutting her losses with him, which, in turn, makes Andy second-guess his separation from his sister. Meanwhile, Reed must keep a huge secret from the group and Jace picks up the mutants' trail again

Polaris' baby begins to suffer from a deadly illness, leaving the Inner Circle unable to heal her, and prompting them to turn to an unexpected source for help. Meanwhile, Reed and Lauren take an excursion to deliver illegal meds, but Reed's powers complicate their plans. Then, Thunderbird and Blink meet the leader of the Morlocks - a shadowy group of mutants that live underground - who offers to help them find the Inner Circle, but the two factions may not have the same goals.

In attempts to stabilize Reed's powers, Reed, Caitlin and Lauren meet with Dr. Risman, a doctor who treats patients with debilitating X genes. However, Lauren finds out Dr. Risman is not who she thought she was. Following the Inner Circle's bank ambush, Thunderbird and Blink continue to work together to chase them down. Meanwhile, Polaris reflects on her troubled relationship with her father and takes steps to protect Dawn from the Inner Circle.

Lauren can't access her powers when it matters most, leaving Caitlin to take matters into her own hands when they are being pursued. Reeva finally reveals her plan for the Inner Circle and it's darker than anyone could have imagined. Meanwhile, Jace confronts some regrets about his actions and is torn between pressures from the Purifiers and his own beliefs.

Reeva prepares to carry out her plan for the Inner Circle, but not everyone is on the same page. Reed struggles to manage his powers and the family realizes that without enough of the serum, it's only a matter of time before he completely loses control. Frustrated and egged on by Benedict Ryan, Jace and the Purifiers are on the attack, but for Jace, things aren't as black and white as they once were.