Carter, Daniel and Teal'c return from a mission claiming that O'Neill and a new addition to the team named Lieutenant Tyler are stranded on the planet under heavy enemy fire, but no one else at the SGC has any recollection of a Lt. Tyler existing. As O'Neill and the mysterious Tyler wait to be rescued, Colonel Simmons arrives to conduct an external investigation into the rest of the team.
"The Fifth Man" provides examples of the following tropes:
- Anti-Air: O'Neill uses a grenade launcher to shoot down a Death Glider.
- BFG: At the end of the episode, Teal'c takes the cannon version of the staff weapon from the downed Death Glider and uses it as his own personal weapon.
- Backstory Invader: Tyler eventually comes clean that he's an alien who secretes a pheromone that makes one perceive them as a familiar (even if fictitious) figure they have always known. Since he was running from their mutual enemies they let him go. In later episodes, they use the memory-altering compound to infiltrate enemy organizations.
- Big Damn Heroes: Carter, Daniel and Teal'c return to the planet just in time to save O'Neill and Tyler when they're cornered and out of ammo.
- Conflicting Loyalty: Simmons accuses Daniel of this as a result of Sarah's status as a host, claiming that his judgement is clouded by his emotions.
- Continuity Nod: Daniel suggests that he, Carter and Teal'c might have somehow traveled to an alternate reality where Tyler doesn't exist, as the situation bears some resemblance to the time he went through the quantum mirror and nobody on the alternate Earth knew who he was.
- Fake Memories: The team all have fake memories of Tyler being on the team going back several weeks. Fraiser also has a brief recollection of him after she accidentally comes into contact with the residue left on Teal'c's jacket.
- HeelFace Revolving Door: Simmons uses Teal'c's recent brainwashing by Apophis in order to accuse him of this.Simmons: Teal'c changes sides more often than I change the oil in my car.
- Heroic Sacrifice: Defied; Tyler tries to pull one of these by leading the Jaffa away so that the team can go through the gate unhindered, but they refuse to leave him behind and go back to rescue him instead.
- Lie to the Beholder: Tyler's race, the Reol, naturally produce a substance that can make them appear as members of another race as part of an inbuilt defense mechanism.
- Line-of-Sight Alias: Lt Tyler apparently got his name from the M.R.E package that showed it came from Tyler, Texas. He later stated to O'Neill that he was from Texas, but his family moved around a lot.
- Major Injury Underreaction: Tyler reacts to the gaping wound in his leg by saying that it's "not a problem", though this is somewhat justified by the end as it's revealed that he has advanced healing capabilities.
- No One Gets Left Behind: A major theme of the episode; Carter, Daniel and Teal'c are adamant about returning to the planet to rescue O'Neill and Tyler, while O'Neill refuses to leave Tyler to save himself, going back for him even after the reveal that he's not a member of the team.
- Obstructive Bureaucrat: Simmons is revealed to have been keeping tabs on the SGC and uses this latest incident as an excuse to carry out a formal investigation of Daniel, Carter and Teal'c.
- Running Gag: O'Neill offers to take Tyler fishing.
- Weirdness Magnet: Lampshaded while Simmons is interviewing Carter.