“Show and Tell” is episode twenty of season two of Stargate SG-1.
There is an unscheduled activation of the stargate and the iris opens without permission and a child steps through. He is neither Goa’uld nor Jaffa, and says that he has a warning. The boy knows things that he says his mother told him, who he indicates is present, but there is no-one there.
The warning is that there is another threat, this time from a race called the Reetu. The Reetu were attacked by the Goa’uld. In retaliation, a faction of them have decided to wipe out humanity, and all other suitable races, to prevent them from being used as hosts by the Goa’uld. Which is a bit harsh on all the potential hosts.
The Reetu are an invisible species. Without a means of detection, they pose a serious threat. The Tok’ra, in the person of Carter’s father after he joined with one of them, are called in to assist with the detection.