As part of an officer exchange program, Benzite Ensign Mendon is brought aboard the Federation starship Enterprise.
Acting Ensign Wesley Crusher mistakes Mendon for Mordock, another Benzite who took the Starfleet entrance examination with him (in "Coming of Age").
Captain Picard suggests a similar officer exchange with the Klingons, and Commander Riker readily volunteers.
Before the ships move away, Mendon scans the Klingon vessel and discovers a patch of strange organic material on its hull but does not bring it to Picard's attention.
Riker assumes command of the Pagh, decloaks and demands that the Enterprise surrender, which Picard agrees to, disgracing Kargan further.
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.
[1] Den of Geek says, in their 2013 review of this episode, "Competent, engaging, full of heart, humour and character interplay.