Star Trek: Deep Space Nine: Season 3, Episode 26

3 - 26: The Adversary

When a destabilizing coup takes places on the Tzenkethi homeworld, a newly promoted Captain Sisko is ordered to take the Defiant on a border patrol to evaluate the situation. However, soon after departing DS9 the Defiant develops a series of malfunctions, leading Sisko to consider that they may have a saboteur onboard.