The station is full of Klingons who have arrived to help Starfleet fight the Dominion. They are led by General Martok, who is not being open with Captain Sisko about the Klingons' plans and motives. Sisko calls on the one officer in Starfleet who has perhaps the most experience in dealing with Klingons: Worf.
Worf mingles amongst the Klingons on the station and eventually extracts some information. Despite having inform on his own people to follow orders, he delivers the information to Sisko. The Klingons are planning to attack Cardassia. They believe that the Cardassian government has been infiltrated by the Dominion. They have no proof of this and Sisko tries to convince General Martok it is a bad idea. Martok rejects his pleas and leads the Klingon fleet toward Cardassia.
When Garak notifies Dukat of the impending attack on his world, the Cardassians are able to scrape together a defense. The Federation condemns the attack on Cardassia, and the Klingons promptly withdraw from the Khitomer Accords, the peace treaty between the Federation and the Klingon Empire. Chancellor Gowron, leader of the Klingon Empire, gives Worf a choice: he may join the Klingons, or be a dishonorable traitor to his people.
Worf chooses to perform his duty as a Starfleet officer. Gowron declares his entire family dishonored. When Worf tries to put in his resignation from Starfleet Sisko refuses to grant it. So Worf joins the Deep Space Nine crew aboard the Defiant as they head off to come between the Klingons and the Cardassians. The Defiant pulls up to the battle zone and beams the Cardassian council from its crippled ship. They race back to Deep Space Nine as a fleet of Klingon ships arrives, armed and ready to destroy the station. General Martok demands the Cardassian council be handed over, but they have been tested and proved not to be Changelings. Enraged, the Klingons open fire on Deep Space Nine.
The station, which has been recently armed with improved weapons including 5000 photon torpedoes which the klingons think is a bluff, defends itself well but still takes heavy damage. Klingon warriors beam aboard and attack the crew at Ops with bat'leths. Losses are great on both sides. When several Starfleet vessels arrive, Sisko argues with Gowron that this situation is the best their mutual enemy, the Dominion, could hope for. With the most powerful Alpha Quadrant forces at war with each other, the Dominion will be free to sweep in and take over. A furious Gowron agrees and the Klingons stand down, but not before warning Sisko; "You have sided against us in battle, and this, we do not forgive - or forget!"
The Klingons still refuse to give up the Cardassian territories they acquired in battle, meaning they intend to stay right where they are for the time being. With much work to do in the near future, Worf agrees to stay on Deep Space Nine.