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.