As part of an officer exchange program, Commander Riker of the Federation starship Enterprise is to be temporarily assigned to the Klingon ship IKS Pagh while the Benzite Ensign Mendon is brought aboard the Enterprise. As the Enterprise rendezvous with the Pagh, young Acting Ensign Wesley Crusher mistakes Mendon for Mordock, another Benzite who took the Starfleet entrance examination with him (cf. "Coming of Age"). Crusher is quick to apologize and makes friends with Mendon. Riker, who has learned some of Klingon tradition from Lt. Worf, readily accepts his role as the Klingon First Officer, using a show of force when his authority is challenged, pleasing the Pagh captain, Kargan. Before the ships move away, Mendon discovers a patch of strange material on the Klingon ship, but does not bring it to Captain Picard's attention.
Worf soon discovers a similar patch on the Enterprise hull, identifying it as a lifeform reacting with the hull. Mendon reveals that he knew of the same material on the Klingon ship. When asked why he didn't mention this before, he reveals that on Benzite ships, it is considered improper to bring up a problem before you have a solution. Picard chastises him for withholding the information, and orders the Enterprise to find the Pagh, as the organism will be more damaging to the older ship. Mendon, Crusher, and the rest of the crew discover the means to dislodge the organism safely while en route.
Aboard the Pagh, Kargan's crew discovers the patch, and Kargan accuses Riker of it resulting from a Federation weapon, noting that the Enterprise had heavily scanned the area during their rendezvous. The Pagh cloaks and remains in communication silence as the Enterprise nears. Even when the Enterprise sends a message with how to remove the organism, Riker is unable to convince Kargan to disengage, leading to a power struggle aboard the Pagh, and a standoff between the Enterprise and the Pagh. Riker prepares to use a transponder he got from Worf before being brought aboard, but Kargan takes it from him, believing it to be a weapon. When Kargan activates it, he finds himself transported aboard the Enterprise's bridge and held at bay by Worf. Under Riker's command, the Pagh decloaks and demands that the Enterprise surrender, which Picard agrees to, serving to disgrace Kargan further. The Enterprise cleans the organism from the Klingon ship, and Kargan is returned. Riker allows Kargan to hit him and order him off the ship to allow the Klingon to regain some of his dignity before the Pagh departs.
Brian Thompson was cast as the Klingon Lieutenant Klag. He had some difficulties during the audition process as the producers originally sought to have the actor portraying that part wear the same costume created for Christopher Lloyd in Star Trek III: The Search for Spock. His audition was originally cancelled, as he later explained, "I phoned Herb Tobias, who was the head of the agency at the time, and told him what happened. I said, 'Herb, would you mind calling the Star Trek office and figuring out a tactful way of asking them, Who casts this program? The directors and producers or the wardrobe department?'". Thompson praised the work of Jonathan Frakes, saying he is a "true professional and I loved working with him."