After returning from a mission, the team is informed that there has been a dangerous chemical spill on base and taken to the infirmary; sometime later, Carter and Teal'c wake up to find the SGC in the midst of a full-scale infiltration by alien forces. Carter manages to escape, and with nowhere else to go, turns to Maybourne for assistance.
"Foothold" provides examples of the following tropes:
- Alien Blood: The aliens are shown to bleed some kind of viscous pearlescent purple substance.
- Alien Invasion: Of the "infiltration" variety. It's revealed at the end of the episode that the aliens came through the gate already impersonating another SG team and slowly took over the rest of the base personnel from there.
- Armed Altruism: As soon as Jack O'Neill frees Major Davis, he gives him his pistol.
- Artistic License – Chemistry: The aliens claim a chemical spill of tetrachloroethylene in the SGC facility to help cover, claiming exposure to the chemical can cause delusions and hallucinations. Tetrachloroethylene is commonly used in cleaning solutions, and it is not known to cause hallucinations.
- All Webbed Up/Unwilling Suspension: Those taken captive are strung up from the ceiling in some sort of web-like material so that the aliens can impersonate them unimpeded.
- Capture and Replicate: The invaders imprison personnel and use them to create holographic disguises. The prisoners are kept alive to allow the aliens to mimic their normal behavior via technobabble.
- Diagnosis: Knowing Too Much: What the aliens impersonating the SG-1 personnel attempt, in a variation, to do with Carter and Teal'c. Alien!Fraser claims that Teal'c went berserk because of the chemical spill, and they had to restrain him for his own safety. Carter, after trying to alert Col. Maybourne, is met by two of the aliens impersonating Daniel and O'Neill, who also make the same claim to her, that she has been affected by the chemical.
- Glamor Failure: Carter discovers that a certain sound frequency will interrupt the holographic projectors used by the aliens, allowing them to be seen for what they really are.
- Hidden Depths: Colonel Maybourne starts out being his typical Obstructive Bureaucrat self, but after he witnesses concrete evidence that Carter is telling the truth about the invasion, he immediately becomes much more helpful. He even leads the spec ops team that retakes Cheyenne Mountain at the end. There's a reason the NID entrusts him with so many black operations, it would seem.
- Holographic Disguise: The aliens use this to disguise themselves as the SGC personnel. Carter later uses one of the devices to impersonate the alien impersonating Daniel, allowing her to sneak back onto the base.
- Humanoid Aliens: The aliens are vaguely humanoid in shape and size, but are otherwise not remotely human looking at all.
- Impostor-Exposing Test: Maybourne cuts his hand in front of Carter to prove to her that his blood is human. It had earlier been established to the audience that the alien blood also looks red provided their holographic disguise is still in effect, so this demonstration actually doesn't prove anything, but neither of them know that.
- Instant Sedation: When the team is taken to the infirmary at the beginning of the episode, "Fraiser" gives them a shot that knocks them out almost instantly.
- One-Man Army: Teal'c manages to take out about half the base before he's finally captured.
- Out-of-Character Alert: A downplayed example; Carter seems to forget that she's still impersonating Daniel when she finds O'Neill, who is noticeably thrown by her calling him "sir" and "Colonel". Once she realizes what she's done, however, she is able to quickly remove the disguise and reveal her true identity.
- Overt Rendezvous: Carter has Maybourne meet her in a very public outdoor cafe to discuss an ongoing alien invasion.
- Pet the Dog: Maybourne gets a moment at the end where he gives Carter all credit for saving the day and commends her actions.
- Psychic Link: The aliens have technology that maintains a psychic connection with the person they're impersonating, allowing them to convincingly copy mannerisms and giving them access to all the knowledge of the person they're supposed to be.
- "The Reason You Suck" Speech: Carter gets frustrated with Maybourne's initial refusal to believe her and isn't shy about letting him know exactly what she thinks of him.Carter: Maybourne, you are an idiot every day of the week. Why couldn't you just take one day off?!
- Self-Destruct Mechanism: The aliens blow themselves up once they realize their plans have been thwarted.
- Teeth-Clenched Teamwork: Between Carter and Maybourne. She clearly isn't thrilled about having to contact him, but since her own chain of command is compromised she doesn't know who else to turn to.
- Would Hit a Girl: O'Neill sucker-punches the alien impersonating Fraiser, though he's clearly unnerved about doing so.O'Neill: That was weird.
