“Vortex” is episode twelve of season one of Star Trek: Deep Space Nine.
A visitor from the other side of the wormhole gets involved in one of Quark’s shady deals, and kills one of the others there, who swears revenge on him. Odo arrests the alien, who calls Odo a Changeling.
It appears that he has heard of Odo’s race before, and he also claims to have met them too, offering to lead Odo to where they are, and demonstrating a peculiar item as proof, said item being, according to Doctor Bashir, the closest thing found to Odo so far.
The man is being taken to his homeworld, where he is wanted for crimes, but he talks to Odo about the Changelings.
This is the first time Odo, or anyone in the Alpha Quadrant, has heard anything about people like Odo. The man is a liar, thief and killer, but is some of what he’s saying the truth?