The body of a murdered federal agent is found, and Five-O is contacted by the government agency that sent him to Hawaii, which also sends another agent, Glen Fallon, to Hawaii to assist Five-O. McGarrett and Five-O discover that the murdered man was investigating the possibility of a "sleeper" agent within the March Foundation, a think-tank funded by the U.S. government. The evidence uncovered by McGarrett and Fallon begins to suggest that the Foundation's chief, Dr. Rathman, is the "sleeper" that the murdered agent was looking for.

Four stabbings have taken place in Honolulu, but Five-O hasn't been able to come up with anything linking the victims, or any consistent pattern among the killings except the murder weapon used. Then McGarrett is unexpectedly visited by Agnes DuBois, a young Englishwoman whose profession is preparing horoscopes. She claims that all of the victims shared certain astrological characteristics. Though McGarrett and the other members of Five-O are initially skeptical, they become intrigued when she correctly predicts the time and place of a fifth murder.

A diplomatic courier with a briefcase chained to his wrist is hypnotized and programmed to rig the case with a bomb. When Danno goes undercover to ferret out the assassination ring, he himself is brainwashed and programmed to kill McGarrett.

Once a juror, always a cop ... McGarrett, serving on a murder-trial jury, is the only holdout for acquittal when the scenario for the "locked-room" killing, which points to the electronics-expert defendant, doesn't match any physical evidence, so he contacts Five-O and asks them to reopen the case.

A woman tries to get her two-bit hustler boyfriend to quit while the quitting is good after he joins a retired Asian general's crime syndicate.

McGarrett tries to thwart a plot by Neo-Nazis to assassinate a popular political candidate.

The King Tutankhamun artifacts are on display in Honolulu, thanks to the efforts of art patron Alicia Warren. While the exhibit is open, however, a man rushes in and smashes a display case, stealing Tutankhamun's gold death mask. McGarrett quickly recovers the item, but then discovers that the recovered mask is a forgery that has been switched for the real mask. McGarrett determines that there are several suspects in the theft, including the curator of the museum where the exhibits were displayed - and Alicia Warren herself.

A small-time Chinese criminal in prison has big ties to the Chinatown mob, which is gearing up for a fight with a rival gang. McGarrett promises the young man a pardon in exchange for his help in thwarting the mob war, and then carries out an elaborate escape and phony assassination attempt to convince the gangsters that their buddy is still on the wrong side of the law.

Danny helps a past girlfriend of his who is now a petty crook, but doesn't reckon on her taking up with a murderous bank robber and being an accessory to his crimes.

A member of the Canadian Parliament is killed in a hit-and-run accident in a remote area, and Five-O discovers that he was apparently on Oahu to meet the one person whom he knew there, a stewardess named Linda Forbes. McGarrett's initial attempt to question Linda is rebuffed by her uptight sister, Diana, though he eventually meets with Linda and finds that she is virtually the exact opposite of her sister. But as the evidence mounts that Linda had something to do with the hit-and-run death, McGarrett is forced to examine what he increasingly finds is the odd relationship between the two sisters.

A distraught man comes on stage during a live TV broadcast by a charismatic preacher and shoves an unloaded gun in his face, demanding the preacher account for the death of the assailant's wife, who was a member of his congregation. When Five-O investigates, they find the charges may be well-founded and the preacher may be heading a cult of brainwashed people to get at their assets.

The Hawaiian "kumu" mob, first introduced in "A Death in the Family" (episode #10.24) returns with a new boss named Tony Alika (Ross Martin), and their first order of business is to kill the head of the Hawaiian music mafia and muscle in on a promising new singer, played by real-life singing star Yvonne Elliman.

To get out from the thumb of the Hawaiian kumu mob, the manager of a promising singer tries to cut a deal with a mainland gangster to get them to buy up her contract, leading to the threat of a vicious mob war.

The vault in a relatively new bank in a shopping center is broken into, yet there appears to be no way that the thieves could have gotten into and out of the vault - there are no signs of forced entry and the alarm was never triggered. But McGarrett slowly begins to suspect Matthew Meighan, the developer of the shopping center where the vault is located - especially when the Five-O team uncovers evidence that similar burglaries have occurred at three other banks on the mainland.

Author Millicent Shand, a friend of Gov. Jameson, is in Hawaii to promote her new novel based on her adventures. She approaches McGarrett and the governor again, this time about her niece, Carole, whose husband disappeared while scuba diving off Honolulu. Shand initially believes that Carole's husband is still alive and is conspiring with spiritualist Sebastian Rolande, whom Shand believes is a phony, to defraud her niece out of half a million dollars. Shand's initial attempts to debunk Rolande's talents as a psychic fail, but she remains determined to prove that he is a criminal.

A spoiled heiress tries to smuggle her dog into Hawaii in defiance of quarantine regulations, and her subsequent romance by a jewel thief who drugs her in order to dognap the pooch and place the stolen jewel in the dog's collar -- only to lose it to the pound.

When Tony Alika's guns shoot out a tire on a police car, resulting in the death of a cop and the near-death of Duke, the whole crime is captured on film by a free-lance photographer (a "stringer" in the trade), who goes to Alika and tries to blackmail him and a mainland hood.

An ex-cop turned private eye murders pimps in order to rescue their girls from lives of prostitution, particularly a youngster he's formed a special bond with.

McGarrett goes along with an old Navy buddy when the latter "investigates" a doctor who may have prescribed steroids to the Navy man's son with lethal aftereffects, but Danno and a fitness instructor find evidence which implicates another person entirely.

A serial killer brazenly tries to sell the rights to his story to a greedy book publisher, and then targets the publisher's daughter as his next victim after being turned down. The killer is the last person Danno books.

An American drug smuggler, who faces death by hanging if arrested by Singapore authorities, escapes McGarrett's grip quite literally by sliding along a tram car cable, then fleeing to one of the tiny islands that surround the main island. A Malay drug lord, stiffed by the American, wants his scalp and kidnaps his wife to force him to come out into the open. The drug lord permits McGarrett to board his boat and to take the wife (who has been forced to ingest cocaine and will soon die if she doesn't get medical help) to a hospital in return for her husband facing death by a bullet. The American, now on the lam, agrees, but secretly instructs his goon squad to create a diversion while he pulls off the ultimate double-cross.