Tom’s mission to destroy the Espheni Power Core is complicated by Lexi’s return. Lexi is forced to take extreme measures to save her father’s life – and her own – after a Beamer malfunction. Meanwhile, the 2nd Mass fears an impending Espheni strike by the scorched Overlord.

With the weight of humanity’s survival on Tom and Lexi’s shoulders, they must defeat Tom’s mortal enemy and find a way to destroy the omnipotent Espheni Power Core. Meanwhile, a new, terrifying Espheni Bomb is dropped onto Chinatown, immobilizing the 2nd Mass and preparing them for Human Skitterizaton.

A recap of the show's first three seasons.

When Charlie is chosen to go on an important trip to Tennessee to see the country’s first plutonium reactor, tensions in the Isaacs’ marriage only deepen when Abby learns that Charlie is accompanied by Helen on his trip. Fritz has a medical emergency, and Frank’s attempt to intervene leads to a shocking discovery.

Masters privately treats a couple for infertility, without revealing his relationship to them. Johnson realizes her work with Masters can be used to help people suffering from sexual dysfunction - but quickly gets in over her head when she attempts to treat Barbara Sanderson on her own. Libby is unnerved after witnessing a hate crime that is then whitewashed by the police.

Lee Drexler returns from his honeymoon, only to be confronted by Cookie Brown, who demands the rights to Marvin's music. Ray arranges the deal, but then Lee reneges on it. Peggy Shaughnessy plants a seed of doubt in Kate's mind about the story that she published; Kate reopens her investigation but is threatened by a pair of Southie thugs on the street.

After discovering the potency of ultraviolet light against the strigoi, Setrakian gains a new and unlikely disciple. A quick trip to a Brooklyn convenience store turns into a deadly all-night siege.