As the Marshmen begin pouring into the Starliner thanks to an infected Romana, the Doctor and Adric hurry to create an antidote but in doing so discover a deep secret about the Marshmen and their connection to Adric's people.
Marshmen invade the Terradonian ship thanks to Romana, who seems to be in communion with them. While the ship's inhabitants flee for their lives, the Doctor discovers a crucial fact why these humanoids can never return to the planet Terradon.—statmanjeff