The post-apocalyptic sci-fi series' fourth season kicks off with the Masons and the 2nd Mass under brutal attack by a menacing creation and a new war machine of the Espheni, the alien race that has invaded Earth. Members of the 2nd Mass are separated, with Tom (Noah Wyle) and others sent to an Espheni ghetto, prompting Anne (Moon Bloodgood) and various survivors to launch a mission to find, rescue and reunite the group. Meanwhile, Lexi (Scarlett Byrne) has grown from a girl to a young woman with ever-expanding and potentially dangerous powers. In other developments, Tom and the 2nd Mass work with the Volm, an alien species and enemy of the Espheni, to prevent an invasion, leading to an all-out battle. Later, the heroic group embarks on a perilous quest to destroy the all-powerful Espheni power core.