The Looking Glass War

Summary
A badly damaged starship arrives unannounced through the wormhole. It is another U.S.S. Defiant, piloted by Curzon Dax. Arriving from a parallel dimension, Curzon needs Odo's help to prevent a war between the Federation and the Dominion. In his dimension, the Dominion invaded and were turned back after heavy losses and only after the Jem'Hadar had taken hostages, including Sisko, Kira, and Bashir. With O'Brien's help, Curzon searched many dimensions until he found one with Odo, because the flashpoint that caused his war was Odo's murder. Sisko denies Curzon's request. Curzon kidnaps Odo and escapes to his dimension in the undamaged Defiant. Sisko and his crew repair Curzon's defiant and follow Curzon for a confrontation with the Jem'Hadar and themselves.