Mariner suspects that Boimler's new girlfriend Barbara is not what she seems, while Rutherford and Tendi tour the USS Vancouver.
The Cerritos assists the USS Vancouver in breaking up an unstable moon. Boimler introduces Mariner to his girlfriend Barbara, who serves on the Vancouver, and Mariner becomes convinced that something about the relationship is wrong and sets out to prove it.
Rutherford and Tendi tour the Vancouver, where Lieutenant Commander Docent pits them against each other and demands that one transfer to his ship, later admitting he wanted out of the Vancouver's dangerous missions. Mariner discovers an alien parasite attached to Boimler that has been influencing Barbara and removes it. Barbara then breaks up with Boimler but becomes friends with Mariner.