TOS Season 3 Episode 2 The Enterprise Incident

Plot

Captain Kirk orders the Federation starship USS Enterprise into Romulan space without any authorization from Starfleet of which the bridge crew are aware. Romulan vessels intercept the Enterprise and Kirk negotiates an hour's time to consider surrendering his ship. Kirk, along with the Vulcan First Officer Spock are then invited aboard the Romulan flagship.
Once aboard the Romulan ship, Kirk and Spock are taken before a female commander who demands an explanation for their intrusion into Romulan space. Kirk claims that instrument failure caused the ship to stray off course, but Spock divulges that the captain ordered entry into Romulan space, and asserts that he is insane. Romulan guards lead Kirk to their brig.
Alone with Spock in her quarters, the commander questions Spock about his career. She argues that humans may have shown their disregard for his talents and capabilities by not giving him command of a ship, but the Romulans, if he were willing, would not make that mistake.
In the Romulan brig, Kirk injures himself by lunging against the force field door. Chief Medical Officer Dr. McCoy is summoned from the Enterprise to attend to him. The commander asks McCoy to confirm Spock's characterization of the captain as mentally incompetent and McCoy does so, whereupon the commander calls on Spock to assume command of the Enterprise. Kirk, calling Spock a traitor, attacks him, and Spock defends himself using what he calls the "Vulcan death grip". Kirk slumps to the floor, and McCoy declares him dead.
Back on the Enterprise, Kirk awakens from the state of suspension brought on by the so-called death grip. His apparent insanity, the unauthorized venture into Romulan space, and Spock's betrayal have all been part of a secret Federation plan to steal the Romulan cloaking device. Kirk orders McCoy to perform surgery to give him Romulan features and then transports back to the Romulan vessel disguised as one of their officers.
Meanwhile, Spock and the commander dine in her quarters, and their conversation grows intimate. When the commander goes to change her attire, Spock directs Kirk, via communicator, to the location of the cloaking device. His signal is discovered and tracked, and Spock surrenders himself to the Romulan officers, but they are too late to prevent Kirk from stealing the cloaking device and returning with it to the Enterprise.
Chief Engineer Scott attempts to adapt the Romulan cloaking device to the Enterprise while Ensign Chekov succeeds in distinguishing Spock's life signs from those of the Romulans. Both Spock and the Romulan commander are beamed to the Enterprise where Kirk gives the order to return to Federation space. The pursuing Romulans are ready to fire upon them as Scott successfully activates the cloak and the Enterprise vanishes before their eyes. The now-captured Romulan commander points out to Spock that any advantage the Federation gains from studying the new cloaking device model will be temporary, as before long the Romulans will learn to circumvent it. Spock admits that military secrets are the most fleeting of all.