The Fifth Man

The Fifth Man is the fourth episode of the fifth season of Stargate SG-1.

Plot
Under attack from Jaffa on P7S-441, SG-1 attempts to escape through the Stargate. However, Lt. Tyler, who is with them, is injured and Col. O'Neill stays behind to help him while Major Carter, Dr. Jackson and Teal'c go for reinforcements. Upon arriving at Stargate Command, the three learn that there is no Lt. Tyler. As the members of SG-1 seem to have false memories, they are not allowed to go back through and are sent to the infirmary. After the evaluation Carter hacks into the base computer system and to their surprise finds out, that there really never was a Lt. Tyler. However they find out that a mysterious 'User 4574' has been looking through the SGC files for their past missions.

Back at the planet O'Neill and Tyler hide in an old stone structure. Soon a Death glider finds them, and although O'Neill is able to bring it down with a grenade launcher, their position is already compromised, and he begins setting up defenses as Tyler is still incapacitated.

Later Col. Frank Simmons from the Pentagon arrives to question the members of SG-1 and General Hammond begins to suspect that he might be involved with the NID. During the inquiry Simmons reveals that he doesn't like SG-1 at all, accusing Daniel of sympathising with the ones that have been taken as hosts by the Goa'uld and questioning Sam's judgement due to her romantic encounters with Niam and Orlin. He also suggests that Teal'c is still loyal to his old masters but Teal'c counters this by pointing out that he would have killed Simmons where he sat if such was the case. Carter in turn confronts him about User 4574, who Simmons apparently is. In the meantime Dr. Fraiser examines the clothing SG-1 wore on the mission and she comes in contact with an alien powder, giving her a brief memory of seeing Lt. Tyler. The powder is similar to the neurotransmitter acetylcholine (although the chemical structure shown on her computer does not actually look like it) and she concludes that this is what caused SG-1's false memories of Tyler.

On the planet Tyler confesses to O'Neill that he is in fact a Reol named Kaiael and what he has done to them. He also shows him his real form and tells him that his species uses their abilities to survive. When the Goa'uld found out about their abilities they started to hunt the Reol hoping to use their memory-altering abilities for themselves. Nevertheless O'Neill is determined to save both of them.

After Dr. Fraiser determines the effects of the powder, SG-1 gets permission to go rescue O'Neill, despite the objections of Col. Simmons. Carter, Jackson, and Teal'c manage to arrive just in time to help O'Neill finish off the Jaffa, who attacked their position and nearly overwhelmed them. As a large amount of Jaffa are now guarding the Stargate, Daniel stays with Tyler while the others plan to retrieve a Staff cannon from the Death Glider which O'Neill shot down earlier for additional firepower. Feeling responsible for SG-1's trouble, Tyler stuns Daniel with a Zat and flees into the woods to lead the Jaffa away from the gate but is quickly captured. Having succeeded in their plan, SG-1 ambushes the group of Jaffa who are taking Tyler back and kill the remaining Jaffa at the gate.

In the end Kaiael thanks them for their help and tells them that his people will likely bury their Stargate. Back in the SGC Col. Simmons plans on reporting serious problems within SGC, with which he hopes to bring down General Hammond.