The first feature film featuring the original Star Trek crew, and a refitted USS Enterprise.

Synopsis
A mysterious, unbelievably powerful cloud of energy is detected by a deep space communications station (and by some unlucky Klingons) on a course straight for Earth. Just about any vessel or probe that gets near it to scan it is zapped with a monstrously powerful energy probe that both destroys the target completely and digitizes it for reference purposes.

Admiral James Tiberius Kirk brow-beats Starfleet into letting him take command once again of the Enterprise, stealing the center seat from Captain William Decker, on a mission to investigate and hopefully stop the giant ball of angry energy before it reaches Earth.

The Enterprise's refit is rushed to get the ship out the door in time to do anything useful, and two crew members are killed by the transporter in a malfunction, including the science officer. The warp drive is also not properly balanced and hasn't yet been tested when the ship is finally launched.

Immediately after launch, Kirk orders the warp engines tested, much to engineer Montgomery Scott's protests. A wormhole is accidentally created, and nearly destroys the Enterprise, but an asteroid pulled into the wormhole is destroyed with a photon torpedo which disrupts the wormhole and returns the ship to sublight speed.

As engineering works to unwedge the ship's drive systems, Mr. Spock is brought aboard the ship by a Vulcan transport, having recently failed to achieve Kohlinar and sent from his homeworld to seek enlightement among the stars. The crew welcomes him eagerly, and his expertise is instrumental in resolving the warp drive's calibration problems.

With the ship's warp drive problems solved, Enterprise intercepts the alien intruder well before it reaches the Sol system. The ship's attempts to hail the intruder only provoke it to fire. Unlike every other vessel the intruder has ever encountered, the Enterprise's shields are powerful enough to deflect the first shot entirely. Spock discovers the intruder's unorthodox communication methods (and notices it has been attempting to contact them), and signals the floating death ball before its second strike hits the ship.

With communication (of a sort) established, the Enterprise flies into the cloud, and discovers it to be a massive energy field wrapped around an absolutely enormous, highly advanced, ship.

It is eventually discovered that the ship refers to itself as "V'ger", and states that its intention is just to find "the Creator". Unfortunately, V'ger is soon convinced that the humans (actually, every kind of life on Earth) are repressing the Creator, and it casually announces that it will soon wipe the planet clean so it can join with the Creator. Oops.

Just in time to avert disaster, Kirk, Decker, and McCoy discover the guts of this highly advanced ship are comprised of Voyager Six, an ancient NASA probe programmed with a seemingly simple objective: learn all that is learnable. The probe wandered off into deep space, was sucked into a black hole, and emerged to apparently fall into the hands of a sympathetic race of machines that decided they could help it achieve its goal (apparently, their idea of help was to arm it to the teeth and teach it to just vaporize and digitize everything in its path on its way back home).

When the crew tries to prove to V'ger that humans were its creator, it shorts its own ancient radio receiver out to stop them. Decker decides to feed the coded proof to it manually (by hand, by fiddling with its wires), which causes its probe (in the shape of Decker's former lover) to merge with him, thereby helping V'ger achieve its next level of consciousness. Mysteriously, this vaporizes Decker but leaves Enterprise and the Earth (which V'ger was orbiting) unscathed.

Brief Review
This movie was not nearly as awful as folks like to make it out to be. Even the "extended version" which included tons more footage of the V'ger flythrough was enjoyable.

The special effects for the film were absolutely astonishing (remember that the film was made in 1979 next time you see the scenes set in engineering), and the plot, goofy as it was, isn't nearly as hokey as some of the later Trek films were.

The models used were very high quality, the transporter effects were impressive, and the Engineering set remains the most impressive Star Trek set ever constructed and filmed. The warp core's appearance, especially, has yet to be matched by another series (and this includes Voyager's warp core, which just didn't convey the same sense of massive size and unimaginable power that this thing did).

The movie isn't without its faults, namely in its overall pacing (the flythrough scenes just drag on and on), and the very stiff acting (did they drive wooden stakes into the actors' arses or something?). Still, it's not a bad story, the visuals and sounds were wonderful, and the musical score (both defining the overall theme of music used in all the new series of Star Trek, and introducing the theme song that would later be used as the opening music of Star Trek: The Next Generation) was truly wonderful.

Sadly, the recently released director's cut of the movie cuts out several good bits, and inexplicably changes the Enterprise's red alert claxon with an incredibly weak and whimpy siren-like thing, but at least the visuals remain intact.

Next time you watch this movie, before you complain too loudly about the movie, remember that when you moan for more action, you get flops like Star Trek: Nemesis. Of course, as Stavr0 points out, you also sometimes get Star Trek: The Wrath of Khan.

Pseudo_Intellectual points out this film was a personal triumph for fan fiction, as it was a fan, Alan Dean Foster, whose script was produced for the film.