William Shatner's James T. Kirk used a Klingon ship in Star Trek IV: The Voyage Home, but what happened to the vessel after the crew completed their mission is ambiguous. Originally acquired by Kirk in Star Trek III: The Search for Spock, the former crew of the USS Enterprise used the stolen Klingon Bird of Prey to travel back through time in Star Trek IV. After Kirk's crew returned to their present, the Klingon ship could be seen sinking into the San Francisco Bay, while Kirk and his crew were awarded a new ship, the USS Enterprise-A, for their valor in saving Earth.
Kirk managed to steal the Bird of Prey in Star Trek III by outwitting the Klingons. After Kirk destroyed the Enterprise to keep it out of Klingon possession, Commander Kruge demanded the Star Trek Genesis Device and ordered McCoy and the others to be beamed aboard his Bird of Prey. With his crew safe, Kirk battled Kruge to the death and beamed aboard the Bird of Prey with Spock just in time to commandeer the Klingon ship before Genesis exploded. The ship, renamed the HMS Bounty, was then piloted by the former Enterprise crew to bring extinct humpback whales back to the 23rd century.
What Happened To The Bird Of Prey After Star Trek IV
Despite the significance of Kirk's Klingon ship in Star Trek IV, its ultimate fate is not directly addressed in the original film series. Possessing a Klingon Bird of Prey would offer Starfleet a rare chance to study Klingon engineering, including their infamous cloaking device. The Federation in Star Trek had been in conflict with the Klingon Empire since the time of Star Trek: The Original Series, meaning a Klingon ship would be a valuable asset for Starfleet to study.
On the other hand, the Federation was in the middle of tense peace negotiations with the Klingons around the time of Star Trek IV, made more difficult by Kirk's actions in Star Trek III. This leaves several possibilities open for what happened to Kirk's Klingon ship after Star Trek IV. The HMS Bounty could also have been used as a token of peace with the Klingon Empire, given their anger over Kirk from Star Trek III. Of course, this assumes that the Klingon Bird of Prey was retrieved from the San Francisco Bay at all.
Was Kirk's Stolen Bird of Prey Valuable to Starfleet?
While the fate of Kirk's Klingon ship is not directly addressed after Star Trek IV, an unused concept from the Klingon story for Star Trek VI: The Undiscovered Country provides the most likely answer. An early storyboard shows the Klingon Bird of Prey being disassembled by Starfleet, likely to study its weapons and cloaking technology. Despite not being in the completed film, this storyboard comes closest to providing an official explanation for what happened to Kirk's Klingon ship after Star Trek IV. As Mr. Spock would say, seizing the opportunity to study a Klingon ship would be most logical.
This concept would have been thematically appropriate for Star Trek VI, given the focus on the Federation's peace with Klingons. The movie would start with Starfleet studying the Bird of Prey for a strategic advantage, only for the threat of a second Klingon War to finally come to an end by the story's conclusion. Kirk's stolen Bird of Prey may have been valuable to Starfleet, but Kirk himself would have agreed that peace was even more valuable, even if that meant handing over the stolen Bird of Prey after Star Trek IV: The Voyage Home.