Mission director Vera Herrick is trapped in a loveless marriage with military hero Colonel Silas Herrick. Silas returns to the planet of Rexor IV to obtain a substance needed to process Earth's toxic atmosphere, but comes under attack from the local Rexorians. When their ship returns on autopilot, Silas has apparently survived, and Vera finds him uncharacteristically kind and considerate. The couple are arrested by the State, who believe that Silas has been taken over by a Rexorian consciousness.
- After the End: It isn't clear what happened to make Earth a Crapsack World ruled by an oppressive government with a Fantastic Caste System, but the fact that breathable air is a valuable commodity worth invading other planets for means it qualifies as at least a Class 1 Apocalypse How.
- Alien Invasion: Humans are doing this to Rexor, due to it having the resources they need to clean Earth's atmosphere. At the end, the Rexorians appear to be successfully striking back.
- Ambiguous Situation: It's never revealed to the audience whether the Rexorian who replaced Silas was a soldier in the first place or ever intended to act as a spy/saboteur on behalf of his people's government (though he would obviously be justified if he did) or, if as in the original Philip K. Dick story, he was just a refugee happy to have survived by chance. His eventual willingness to let himself be executed to protect Vera points toward the latter.
- Bureaucratically Arranged Marriage: The State apparently pairs people as couples due to a "procreation mandate", which is how Vera and Silas were put together, and this explains why their marriage was quite cold.
- Energy Beings: The Rexorians.
- Fantastic Caste System: As a war hero and a commanding officer in the military, Col. Silas Herrick is a "Level 1 Citizen", entitling him to such amenities as a large apartment, a steady supply of fresh food and breathable air, and actual psychological treatment for his PTSD (which he does not avail himself of), as well as the formality of a trial when suspected of treason rather than immediate summary execution.
- A Father to His Men: His abominable home life aside, Col. Herrick earned his medals for his reputation for staying behind to personally drag his men to safety one by one rather than leave anyone behind. This is what he was doing when he and poor Pvt. Matthews get body-snatched.
- Fridge Logic: At the trial, the human authorities claim to have no prior in-person encounters with Rexorian metamorphs and no clear understanding what they're capable of — yet they know of the existence of metamorphs and do have a clear understanding of the mechanics of how they replace people. Given the nature of the government in this story the discrepancy may be intentional.
- Jerkass: Colonel Silas Herrick before being possessed.
- Jerkass Has a Point: The real Silas is a piece of work and his outright emotional abuse can't be excused. But he is apparently correct that Earth will die without a steady supply of chemicals to synthesize clean air, and therefore the stakes of the Rexorian invasion are very high. And no one can argue that he is a war hero and A Father to His Men. Much of his behavior may stem from untreated trauma from the battles he's been through.
- Mars Needs Water: Inverted Trope. It's the humans that are stealing elements from Rexor to fix Earth's atmosphere. A big clue that Humans Are the Real Monsters in this story.
- Ms. Fanservice: Vera is shown in an extended sequence seeking solace from her Sexless Marriage by having an anonymous three-way with a younger couple, that does little to advance the plot. Later she starts having sex with Silas (or his impersonator) and is shown topless both times.
- Not Himself: What allows the main character to realize her husband is not human anymore.
- Patrick Stewart Speech: Sort of a subversion. The prosecution in the trial claims that Rexorians are incapable of compassion and self-sacrifice. Vera points out that her husband's willingness to sacrifice his life for her then proves that he is human.
- Puppeteer Parasite: Rexorian metamorphs apparently have this ability. It's implied to be a permanent merging.
- Sense Freak: Implied, thanks to the Rexorians initially being bodiless balls of light. The new Silas definitely has an appreciation for food and for the culinary arts the old one didn't, to say nothing of his newfound sexual appreciation for his wife.
- Sexless Marriage: Vera and Silas's marriage is implied to be one before his return. After this, he starts showing sexual interest in her, which greatly surprises Vera. After they have sex, she says he never before touched her that way, so apparently they hadn't even consummated it.
- Spiritual Antithesis: To the classical body-snatcher stories, such as Invasion of the Body Snatchers (1956). Vera much prefers the possessed Silas, who is much more humane than her "real" husband. This notably serves as such for a different episode of this very show, "The Father-Thing", which is based on a Philip K. Dick story that predates and inspired Invasion of the Body Snatchers. Interestingly, the selfsame behavior that convinces the main character of "The Father-Thing" that his father has been replaced — his father's sudden desire to rekindle his relationship with his wife — is what saves the new Silas from execution here.
- Talking in Your Sleep: How the Rexorian who replaced Pvt. Matthews got caught.
- There Are No Therapists: Subverted. Vera reminds Silas he is entitled to "Level 1" psychiatric care, but he's never used it — the real Silas out of pride, the new Silas out of fear of detection.
