The Hawaiian kumu mob attempts to take over a resort-workers union, to the fury of native Hawaiians who vengefully infiltrate and smash underground activities in lawless fashion. Meanwhile, Boston ex-cop James Carew trails a mainland gangster - who is providing aid and comfort to the kumu - to Hawaii in order to get information on the murders of his wife and child.
In his last appearance, kumu mobster Tony Alika tries to divert Five-O's attention from his smuggling efforts -- including PCP and a hired killer who murdered Kimo's wife and son -- by sending phony tips to Five-O which make them look "like Keystone Kops" (quoted in a sign at one so-called crime scene) who should be disbanded. This was the only 12th-season episode to be repeated in prime time, as the final telecast on April 26, 1980.
An ex-con out for vengeance against a former District Attorney teams with the D.A.'s daughter, who's out for vengeance against her mom (the D.A.'s divorced wife) by faking his own death (the mom shoots him with a gun loaded with blanks), and then showing up time after time at the mentally unstable mom's favorite hangouts, and then promising to stop if he's paid off from the daughter's trust fund, which he and the daughter intend to share.
Various "owners" of a handgun of unusual design, which was used in a murder six months earlier and then dumped. Two punks use it in a robbery-shooting that leads to one of the thieves also being injured. When media coverage reveals the nature of the still-unsolved murder from before (of a state senator), the leader of the gang tries to reclaim the gun as it passes from hand to hand, and to sell it back to the killer (the son of another state senator who was the victim's political enemy). Several more people are shot as the gun makes its way through the Hawaiian ...
A group fanatics who were plotting to take over a radio station so that they could broadcast their message are pulled over by a police cruiser. They shoot one of the two cops, then decide to take the cruiser and the other cop hostage, and use the police radio as a link. When McGarrett hears of this, he sends Truck and another cop to try and take them but they kill the cop he's with and keep Truck. Steve has to find another way to get to them so he hopes that their associate who was wounded when they took the cruiser and left behind might be able to help them. Steve ...
A college student returns to Hawaii after a 20-year absence (his parents died when he was a little boy and he was adopted), haunted by a series of disturbing flashbacks which seem to make no sense. Things start to come together when a convict gets out of prison after serving 20 years for bank robbery, and some of the student's visions seem to match the robbery scene. The student and the convict confront each other, and before long the convict is dead, his head bashed in. Did the unstable student freak out and murder the convict, did he kill the enraged convict in ...
Kimo and Truck are sent to a remote location to help a scientist who is investigating several mysterious deaths by inhalation. Before long, Truck also shows signs of illness which he attributes to unknowingly violating a native curse (actually, in a subplot, we learn that he had been exposed to plutonium on a previous assignment). The attractive doctor and Kimo hit it off, which makes both of them targets for the murderer, who is secretly harvesting coral from offshore and selling it.
A plastic surgeon's wife is kidnapped, and against his wishes Five-O is brought in to help find her. McGarrett and Kimo are puzzled by the doctor's apparent lack of concern for his wife, and Five-O's investigation of their relationship turns up some equally disturbing facts -- such as how her face was completely different when they lived on the mainland.
Five-O is assigned to provide security for a meeting involving a Middle Eastern sheik from OPEC, who is coming to Honolulu to negotiate with oil magnate John Ellington. McGarrett gets information that someone may be planning to disrupt the meeting, and sends Kimo to infiltrate what turns out to be a school for assassins operated out of an estate on Oahu -- whose best student, Kelsey, is tasked with assassinating the OPEC representative.
A school for wayward girls, run by a friend of Steve McGarrett's, needs $15,000 for overdue taxes or the school will be closed. Former counterfeiter Willie McFee, a flower breeder who also is a general handyman and gardener for the school, decides to make one last print run with a set of near-perfect $20 plates he once crafted - but a mainland crime boss gets word that Willie's plates have surfaced, meaning that he can try to steal them for himself.
The next-to-last show to be filmed features an escaped convict posing as a diplomat from a Far Eastern country in order to get at the country's store of gold bullion in a Honolulu bank. He hires experts with lasers to penetrate the light-beam screen surrounding the bank vault, burn a hole in the vault's floor, heat the gold until it liquefies and drain it through ceramic tubing. McGarrett gets a clue to what's happening when the bank's thermometer (located near the vault) starts to freak out, but he still has to find the thugs' hideout before they solidify the gold ...
Israel's celebrated Nazi-hunter Yuri Bloch has apparently been killed in Honolulu while trying to gather information about a war criminal named Emil Klaus. Knowing the Bloch would only have come to Hawaii if a former Nazi were hiding there, McGarrett and Five-O conduct their own search for Klaus -- while Bloch's partner lies badly wounded in a safe house, and the woman caring for him does what she can to assist while undercover.
During a stopoff on a bird-watching expedition, tourists photograph an old sugar mill. They don't know that the mill is a front for a gold-smuggling operation and that one of their photographs has caught the leader through a window. The leader sends a pack of goons to kill everyone in the tourist party and steal their film. McGarrett and a newspaper journalist, who got hold of the pictures, must try to identify the man in the picture and take down his operation before he gets them.
Paul Burke guest stars as attorney David Lawrence, a political candidate whose promising bid for the U.S. Senate is endangered by the murder of a man who tried to blackmail him. Although Lawrence swears he knows nothing about the murder, he admits that the dead would-be extortionist did have knowledge about him that could have destroyed his political future.
The series concludes with the final showdown between McGarrett and Wo Fat. Three scientists who have disappeared have one thing in common, they all attended a symposium on possible space-based, laser defense systems. McGarrett impersonates a fourth scientist, Dr. Elton Raintree, who attended the same gathering and is soon abducted. Wo Fat is behind it all and wants the scientists to complete their work and produce such a device.