DS9 Season 5 Episode 1 Apocalypse Rising

Plot

Having learned from security chief Odo that a Changeling has taken Gowron's form and is now in control of the Klingon Empire, Captain Benjamin Sisko is ordered by Starfleet Command to expose the impostor. He, Odo, Miles O'Brien, and Worf must infiltrate Ty'Gokor, the headquarters of the Klingon military, disguised as Klingon candidates for the "Order of the Bat'leth". They are given four devices which, when activated, will emit radiation which will force any nearby Changeling to revert to its gelatinous state. Gul Dukat escorts them to Ty'Gokor in his captured Klingon Bird of Prey.
The four arrive at the Hall of Warriors, where the all-night party before the induction ceremony has already begun, and try to blend in. When General Martok, second in command to Gowron, arrives, the four scramble to set up their radiation emitters. Gowron arrives and begins issuing the awards. When Sisko is called to receive his award, he attempts to activate the emitters, but is knocked to the floor by Martok, who has finally recognized them and has them thrown in a security cell.
Once they are alone, Martok confides to Sisko that he too believes that Gowron is an impostor. With the emitters destroyed, the only way to expose Gowron is to kill him. Martok releases Sisko and his group and leads them back to the Hall of Warriors. Worf challenges Gowron to a one-on-one duel. Martok wonders why Sisko does not shoot Gowron outright. Odo observes that Gowron chose to fight Worf in single combat, thus showing Klingon honor, while Changelings do not care for honor; he concludes that the Changeling is not Gowron, but Martok. Just when Worf is about to kill Gowron, Odo reveals the false Martok to the crowd, and the Klingons open fire, quickly destroying the Changeling.
Gowron realizes that Odo was fed false intelligence that Gowron was the Changeling, which would have led to Gowron's assassination by the Federation, allowing the fake Martok, and therefore the Dominion, to gain full control of the Klingon Empire. Gowron agrees to a ceasefire in the war between the Klingons and the Federation, and Sisko and his men are returned safely back to DS9.