“In the Line of Duty” is episode two of season two of Stargate SG-1.
SG-1 and some more team members from the SGC are rescuing some people whose planet has come under a sudden and unexpected Goa’uld attack. Whilst trying to resuscitate one of them, Carter is possessed by something that enters her through her mouth. She starts behaving slightly strangely, which draws a bit of attention, but not that much until Cassandra, the girl who was made into a bomb in “Singularity”, senses something wrong with Sam – she believes that Carter is a Goa’uld (the Goa’uld’s tinkering with Cassandra has left her pretty sensitive to their presence).
It seems Cassandra is correct, but Sam hasn’t been possessed the normal way – or by a normal Goa’uld. The man Carter tried to revive wasn’t the only person possessed by a Goa’uld, and the other is far less friendly.
This episode has the first appearance of the Tok’ra, who appear in many more episodes in the future.