A shapeshifter plot puts the Defiant on a dangerous heading.
Sisko is promoted to captain. An ambassador reports a coup among the Tzenkethi, and Sisko takes the Defiant to show the flag on the border. Systems fail, sabotage is found, and the ambassador turns out to be a Changeling who escapes into the ship and seizes control in an attempt to start a war.
Blood tests are faked, paranoia spreads through the crew, and Sisko readies the ship to self-destruct. Odo fights and kills the infiltrator, the first time a Changeling has harmed one of his own. The coup report proves false, and before dying the infiltrator warns that Changelings are already everywhere.