Summary
Only one Star Trek movie shows the entire cast of the original series together on-screen: Star Trek IV: The Voyage Home.
In the movie, the crew time travels to 1986 to save Earth by retrieving two humpback whales.
The shot of the entire crew together is at the end of the film during a court-martial scene.
In an interesting tidbit shared on Twitter by ZX Geoff Owen (@NCC_17fOrmula1), it is revealed that Star Trek IV: The Voyage Home is the only movie where the entire cast of Star Trek: The Original Series can be seen together on-screen. This unique moment occurs towards the end of the film during a court-martial scene involving the crew of the USS Enterprise. For more information, take a look at the Tweet below:
There are, in fact, other scenes in Star Trek IV where all 7 members of the USS Enterprise's crew are seen together, like the one where Admiral Kirk and his friends explore the streets of San Francisco. However, ZX Geoff Owen's Tweet accurately points out that in the mentioned scene, TOS cast members Grace Lee Whitney as Janice Rand and Majel Barrett-Roddenberry as Christine Chapel are also present.
Why Star Trek IV Is The Only Movie The TOS Cast Is Seen All Together
In all six Star Trek: The Original Series films, the complete USS Enterprise crew is featured. However, due to story logistics and circumstances, they do not appear all together until Star Trek IV. For example, in Star Trek: The Motion Picture, Admiral Kirk gathers the crew to brief them about V'Ger, but Mr. Spock has not yet returned to the Enterprise. Similarly, in Star Trek II: The Wrath of Khan, Scotty remains in Engineering, so he is not present on the Enterprise bridge. Additionally, Captain Sulu spends most of Star Trek VI: The Undiscovered Country on the USS Excelsior.
The crew of the USS Enterprise, consisting of its seven members, are all seen together at the Khitomer Conference in Star Trek VI. Ambassador Sarek also joins them, but Janice Rand and Christine Chapel are not featured in the film. Therefore, the court-martial in Star Trek IV: The Voyage Home remains as the only instance where Kirk's core group of seven, along with Chapel and Rand, are seen together in a Star Trek movie.