Chris Pine takes the bridge as James T Kirk in the prequel, Star Trek
Spock actor Leonard Nimoy set his phasers to stun at a movie screening on Monday when he switched 1982's Wrath of Khan for the new Star Trek film.
Sci-fi enthusiasts in Austin, Texas turned up for the classic film only to find Nimoy introducing the first public showing of director JJ Abrams' prequel.
The film was given a six minute standing ovation and one filmgoer said fans were "crying they were so happy."
Star Trek, the 11th film in the franchise, is due for release on 8 May.
It follows James T Kirk as he graduates from Star Fleet Academy and becomes captain of the starship Enterprise.
Chris Pine takes over the role of Kirk which was played by William Shatner in the 1960s television series and seven of the films.
The prequel had its official world premiere in Sydney, Australia on Tuesday night.
Film company Paramount arranged for Nimoy, along with the film's writers and its producers to unveil the film to hardcore fans first and the audience from Monday's unexpected showing immediately started posting reviews on Twitter.
Spock is played in the new film by both Zachary Quinto and Leonard Nimoy
One fan called it "the best Star Trek movie ever" adding, "yes, it even beat Wrath of Khan".
Another said, "the cast is superb, the story is compelling, the action is exciting". Another simply added: "Just saw the new Star Trek movie and it MELTED MY PANTS!!!!!"
The enthusiasm carried over to the Sydney Opera House premiere, where one viewer reported "there were a lot of moments where the audience spontaneously burst into applause".
Nimoy, who played Vulcan crew member Spock in the television series and several films, has a role in the prequel.
Heroes actor Zachary Quinto, who plays the younger Spock, commented at the premiere: "Leonard and I have become very close in this process and his experience has been unique to him and mine will be unique to me."
The new cast also features Winona Ryder, Eric Bana and, as the younger version of Scotty, British actor Simon Pegg.