On Earth the Klingons are claiming that the death of their Bird of Preys crew at Kirks hands is little less than an act of terrorism, with Kirk co-ordinating the whole Genesis project in an attempt to destroy the Klingon people. Despite the efforts of Ambassador Sarek they claim that there will be no peace between the Federation and the Klingon Empire while Kirk lives.
Meanwhile, on Vulcan Spock is completing his re-training in science and logic after the events of the last film. He is easily able to answer questions about science, history, alien cultures, etc. - but when asked the simple question "how do you feel?" he is at a loss. His mother tells him that as a half human, he has human emotions and these will surface whether he wants them to or not. Spock dismisses the idea, and it becomes clear that this is not quite the Spock we know - he seems to have lost his human side entirely. Kirk and his crew have been on the planet for several months, and although they are under Sareks protection and so immune from prosecution for stealing and later destroying the Enterprise, they vote unanimously to return home and face the consequences of their actions. Using their captured Klingon Bird of prey - now named HMS Bounty by McCoy - they set off for Earth.
Back on Earth, they are tracking the probe as it heads for the planet. Every ship which comes anywhere near the probe is immobilized, and there seems to be nothing they can do to stop its advance. As it reaches Earth the probes transmissions begin to vaporize huge volumes of water, threatening to create cloud cover across the whole planet. The president transmits a planetary distress signal warning all ships to avoid Earth.
On route back home, the Bounty picks up the signal. Spock analyses the probes transmissions and realizes that they are a distorted version of the songs sung by Humpback whales - a species which was hunted to extinction by Humanity centuries before. The crew surmise that the whales were somehow in contact with an alien species, and that this species sent the probe to determine why they lost contact. With no whales left alive there can be no answer to the probe, and so no way to convince it to leave.
Kirk decides that they must travel back in time to retrieve a whale or whales from the past and return it to the present, in the hopes that it will then tell the probe - as McCoy puts it - "what to go do with itself". The Klingon ship isn't an ideal candidate for time travel, but after a rough ride they manage to reach the 20th century successfully. Unfortunately the trip drains the ships dilithium crystals to the point where they are decrystalising and will soon be useless. Spock suggests that they might collect high energy photons from a nuclear reactor on Earth and use them to recrystallise the dilithium. Cloaking the ship they land in Golden Gate park, and split into teams - one to create a whale tank in the ship, one to find the whales, one to locate a reactor and collect the necessary photons.
Kirk and Spock search out the whales, finding a pair named George and Gracie in a huge tank at the Cetacean Institute. Unfortunately Dr Taylor, the institutes assistant director, tells them that the whales are due to be released to the wild soon. Spock mind melds with Gracie, much to the annoyance of Dr. Taylor, and gets her agreement to their plan. As they are on their way home they meet Dr Taylor again and Kirk manages to get a dinner invitation out of her.
McCoy and Scotty locate a factory which manufactures the materials they need for their whale tank. Pretending to be visiting academics studying construction methods, they bribe the factory owner to give them what they need by offering him a little advanced materials technology - rationalizing it by wondering if he didn't invent the stuff in the first place.
Kirks date with Gillian goes reasonably well - she is worried about releasing the whales into the wild because of the danger from hunters, and Kirk tells her he could take them somewhere where this will never be a problem. Although she seems to sense that he is being honest, she still thinks he must be some kind of lunatic - an impression not helped when Kirk admits that he works in outer space! When she drops him off in the middle of the park, Gillian drives away - then stops and turns around only to find that he has mysteriously vanished from sight.
Meanwhile, Chekov and Uhura have managed to locate a nuclear reactor - none other than the aircraft carrier USS Enterprise. They beam into the ship that night to collect the photons, but have trouble beaming back as the Bounty's power reserves are so low. Uhura manages to make it through with the collector, by Chekov is captured by Marines and arrested. He is interrogated by the FBI, who quickly decide that he must have some sort of mental illness. He attempts escape, but as the Marines chase him he falls from the deck of the ship and is badly injured. Back at the ship, Scotty begins to recrystallize the dilithium and get power back.
The next morning, Gillian arrives at the institute to find George and Gracie have gone - they have been shipped out early to avoid the media, and she was not told in case it hurt her feelings. Distraught, she goes to the park and begins running around yelling Kirks name. She soon gets a couple of nasty surprises - she sees a helicopter lowering one of the whale tank walls into the top of the invisible ship, then runs into it herself. Kirk beams her aboard and she tells him about the whales.
Uhura manages to locate Chekov in a local hospital, and the crew set off to rescue him with the help of Gillian. They find him about to undergo surgery, and after fixing his injuries they try to sneak him out of the hospital. After a police pursuit they manage to get out of sight long enough to beam back to the ship.
Kirk convinces Gillian to give him the frequency of the homing signal attached to the whales, and beams aboard - only to have her literally jump on him at the last second so she can tag along. She tells him she has nobody to miss her on Earth, and anyway if they take the whales back with them they will need an expert since nobody in their time knows anything about them. Kirk agrees to let her come along.
The Bounty takes off and heads for the source of the signal, but they detect a whaling ship closing on the pair. It is a race it looks like the whales are going to lose when the whaler fires off a harpoon - only to see it bounce of the hull of the invisible ship. Kirk orders them to decloak, and the whaler turns tail and runs like hell as the ship hovers menacingly above it.
Scotty beams up the whales and they head back into space and slingshot around the sun again to return to their own time, arriving back just before they left. Unfortunately the probes transmissions neutralize the ship as soon as it appears, and it crash-lands in San Francisco bay and begins to sink. Kirk orders the crew to abandon ship while he makes his way to the tank and blows an emergency exit open to release the whales. George begins to sing almost as soon as he is released, and he and the probe exchange information for several minutes. Then the probe shuts its transmitter down and quietly heads away into space, leaving Earth to recover behind it.
With the crisis over the crew are taken to the Federation council to stand for their crimes. The president declares that all charges except one - disobeying orders - are summarily dismissed, and that this charge will only be directed against Admiral Kirk. He is found guilty, reduced in rank to Captain, and ordered to assume command of a Starship as a consequence. A delighted Kirk is congratulated by his friends, including Gillian - who is herself off to serve on a science vessel while she does a little catch-up learning.
Spock meets his father in the chamber; Sarek tells him he may have been in error in opposing his enlistment in Starfleet and asks if there is any message for Spocks mother. Spock asks him to tell her "I feel fine".
The crew head off to Spacedock to pick up their new command. McCoy gloomily predicts that they will be assigned a freighter, while Sulu tells everyone he wants the new Excelsior. The shuttle heads for the Starship... only to pull up above it and reveal a Constitution class Starship behind bearing the name Enterprise and the number NCC 1701-A. Kirk declares "my friends... we've come home".
Nimoys directing is again good. As in ST III he gives almost the entire cast at least one good scene. The only exception is Sulu, and this was not an oversight but rather an unfortunate result of circumstance. Originally Sulu was to have had a scene in which he encounters a young oriental boy, only to realize that it is an ancestor of himself. Unfortunately, the young actor hired for the part was unable to perform on the day and there was no money to do another days shooting, so the scene had to be cut. But the rest of the cast acquit themselves well in their respective scenes.
Where ST III used these scenes for the dramatic stealing of the Enterprise, this is a much more light-hearted movie and most of the scenes are used for comedy. The humour mostly comes from the culture clash between our heroes and present day society, and this works very well. From Scotties overacting at Plexicorp through the scene where the russian Chekov stands in the middle of a street asking passers by and a nearby cop for the location of "nuclear wessels" and up to McCoys disgust at 20th century medicine - "sounds like the goddamned Spanish Inquisition to me", this film is one which will make you laugh out loud more than a few times.
Having said that, humour is not all this film has to offer. The alien probe is both menacing and mysterious, and its final dialogue with the whale has an eerie quality which has survived many viewings. The time travel scenes are well done - most especially, in this scene you can faintly hear dialogue which will later feature in the film proper, as if the characters are experiencing their future actions as they travel back in time.
So much for the good, what of the bad? Well, oddly enough in many ways the good and bad parts are one and the same. In creating a film with mass appeal, Nimoy has by default created one which is a little less appealing for a heavy-duty fan such as myself. Don't get me wrong - I still like ST IV, but for me it just lacks that certain something that sets a Trek film apart from the rest.
Technically, there are a few nits - some minor, some major. Many of the more minor points concern the time travel. At warp speeds the Bounty should have been able to whip around the sun in milliseconds, rather than the half minute or so which it actually takes. There is talk of not even making breakaway speed, yet warp speed is literally hundreds of thousands of times faster than the escape velocity of the sun. Then there is talk of breaking thrusters which will slow the ship down - just shutting down the warp drive will do that just fine. We also see the Bounty go to warp within the Earths atmosphere - while strictly speaking there's nothing in the canon to indicate that this is difficult or dangerous, I am far from alone in finding it a peculiar thing to do.
By far the biggest nit of the film is its whole premise. Supposedly, the whales singing is actually a form of communication with an alien intelligence. But there is absolutely no explanation of just how sound is meant to propagate into space, let alone across countless thousands of light years. Like most nits, it's one which could have been fixed with one or two lines of dialogue - it doesn't even need an explanation, a simple recognition of the fact that the mystery exists would be sufficient.
Overall then, this is a film which I think is very good, but not quite up to the standard of the likes of ST II.