A fleet of Klingon ships is stationed around Deep Space Nine, and the Promenade is swarming with Klingons. However, the warriors aren't their usual antagonistic selves. General Martok, who leads the force, tells Sisko they have come to help the Federation fight the Dominion. Sisko finds this odd, since the Federation neither requested nor expected help. Garak and Odo then have a run-in with Drex, a young Klingon, who later attacks Garak for no apparent reason.
When a freighter is stopped by a Klingon Bird-of-Prey, Sisko confronts its commander, Kaybok. Under orders from the Klingon High Council to assist the Federation alliance, he insists the freighter must be inspected for Changelings. But Sisko, aboard the Defiant, forces him to let the vessel pass. Martok later has Kaybok killed for not carrying out his orders. Since Martok will not explain their motives, Sisko summons Lieutenant Commander Worf, formerly of the U.S.S. Enterprise, hoping the Klingon officer can determine their plans.
Worf starts a fight with Drex, Martok's son, in order to get the general's attention, but Martok won't reveal why the Klingons are on Deep Space Nine. Worf's allies are equally closed-mouthed, but he soon learns the truth from a Klingon officer indebted to his family. Realizing he is betraying his own people, Worf reluctantly informs Sisko that the Klingons are planning a massive invasion of Cardassia. Apparently, a coup on that world has left a civilian regime in power, one which the Klingons insist is backed by the Dominion. But, with no proof to support this belief, Martok is asked by Sisko to call off the invasion. However, the Klingon general instead leads his fleet on a course into Cardassian space.
The Federation orders Sisko to remain neutral until word is heard from the Klingon leader, Gowron, who Worf believes is using the Dominion takeover idea as an excuse to reclaim their race's warrior heritage. Garak is allowed to learn about the dire situation, after which he tips off Gul Dukat on their homeworld, allowing the Cardassians a chance to defend themselves. The Federation condemns the invasion, at which point the Klingon Empire withdraws from their peace treaty. Gowron then comes to Deep Space Nine and offers Worf the chance to redeem himself by joining his people in the assault on Cardassia.