The Merchantman was a small civilian freighter hired by the Klingon agent Valkris in 2285 to deliver stolen Genesis data to Commander Kruge, who then destroyed it with all aboard.
The ship appears at the start of Star Trek III: The Search for Spock. Its crew have carried Valkris to a rendezvous in open space without knowing what her business is, and its captain grows nervous when no one seems to be waiting.
Kruge's Bird-of-Prey decloaks alongside, and Valkris transmits the summary of the Genesis project that she has obtained. When she admits that she has looked at the material herself, Kruge tells her that this is unfortunate. She accepts her fate, he salutes her honour, and the Bird-of-Prey destroys the freighter.
The scene is the first on-screen use of a Klingon cloaking device in the films. The studio model was reused many times afterwards as other cargo ships.