“Defiant” is episode nine of season three of Star Trek: Deep Space Nine.
Major Kira is overworked so Doctor Bashir prescribes her some time off. During this she bumps into the Enterprise‘s Commander Will Riker, and agrees to show him the Defiant.
Only it’s not Will, but Tom Riker, the identical twin created by a transporter accident who was discovered in the Next Generation episode “Second Chances”. Tom Riker sympathises with the Maquis and is stealing the Defiant for them, which he then takes into Cardassian space.
In order to prevent all out war between Cardassia and the Federation, Sisko heads to Cardassia Prime with Gul Dukat, in order to help the latter stop the Defiant. Tom has discovered rumours of certain things happening in Cardassia that the Central Command does not know about, and is following them up. The Obsidian Order is playing their own game.