Seska is revealed as a Cardassian spy.
Voyager's crew finds evidence that someone aboard has been secretly passing technology to the Kazon, and an investigation reveals that Seska, a seemingly loyal crew member, is a Cardassian agent who had been disguised as a Bajoran. She had long been working for her own side.
The episode is a key moment in the first season, because it ends the idea that the Maquis and Starfleet crew were free of internal betrayal. Seska would return as a major recurring villain, and the twist remains one of the first season's best-remembered revelations.