During a party celebrating Sisko's promotion to Captain, Admiral Krajensky takes him aside and quietly reveals that there has been a coup on the Tzenkethi homeworld that could threaten Federation colonists. Sisko and his crew are to take the Defiant and Krajensky to this sector. Soon after departing, they receive a distress signal from a colonist indicating that Barisa Prime is under attack from the Tzenkethi. The signal then goes dead -- the colony has apparently been destroyed. The crew sets course for Barisa Prime, but when Sisko tries to notify Starfleet Command, power fluctuations in the ship's communications circuitry prevent it. O'Brien and Dax go to work on the problem, and discover strange alien devices attached to almost every critical system throughout the ship. They determine that there is a saboteur aboard. Dax scans each member of the crew, and when she gets to Krajensky, he suddenly morphs into a Changeling and escapes.
