Season Three
When SG-1 return from a routine mission, they are ordered to the infirmary. Doctor Fraiser sedates them, using the excuse that the base has been exposed to a chemical leak.
When Sam recovers, she overhears two aliens posing as Fraiser and Hammond.
The SGC has been infiltrated, there's a Foothold situation, Sam escapes and turns to an unlikely ally, Colonel Maybourne..