Al Harrington's first episode as Ben also introduces Duke Lukela and John Manicote as semi-regulars. Manicote launches an investigation of Five-O when Duke, an HPD sergeant who sometimes joins Five-O on investigations, is accused of being on the take. McGarrett does what would be now called an intensive database search, with numerous records on all Five-O team members transferred to projection slides and put up on the screen (if you can freeze-frame or slow your player to catch all of them, there is a wealth of information on the characters -- including McGarrett's birthday, which is in the wrong month!). Convinced that Duke was set up by someone, McGarrett repeats the process with members of Manicote's office and finds that one of the Assistant District Attorneys is a mole planted long before by the mob to discredit the office. Guest star Michael Ansara, playing the mob boss, forsakes his toupee (he's shown swimming) and is very bald.

Champion race car driver Alex Pareno arrives in Hawaii to attempt to break the record for driving up Tantalus Mountain. However, his chief mechanic is bludgeoned to death after catching someone tampering with Pareno's car. Later, a second mechanic is killed while test driving the car. McGarrett eventually discovers plenty of suspects ranging from Pareno's fiancée Angela, who was being blackmailed by an ex-lover, to Pareno himself. However, it is eventually discovered that the killer is someone very close to Pareno.

A sophisticated operation is blackmailing rich and prominent visitors to Hawaii. One of the victims has told Tolliver, a private investigator, about his situation. Tolliver tells his friend that he'll work to get him free of the blackmailers. Meanwhile, a young woman has turned up dead -- which brings Five-O into the case. It turns out she was among numerous women used as bait, but she has become too hot to handle when one of the blackmail victims committed suicide. Tolliver, meanwhile, poses as a rich man to attract the attention of the blackmailers. But he has no intention of ruining the operation. Tolliver intends to take it over.

During a traffic stop in the boonies, the driver slips out a concealed handgun and shoots the HPD officer down in his tracks. Before dying, the cop gets off a couple of rounds at the fleeing car, causing it to crash. McGarrett and Danno visit the killer in the hospital, where he still couldn't care less about murdering a man ("I chopped a cop ... put a pig in a blanket"). The incident preys heavily on both men's minds, particularly Danno's. Later that same night, Danno witnesses a liquor-store robbery where the masked thief takes a shot at him. Danno follows the thief to a house, but loses him on the grounds. When a person steps out appearing to be carrying a pistol, Danno fires -- and finds to his horror that the young man was an innocent kid living at the house -- carrying a soldering iron. As the wounded boy lingers comatose and in critical condition, McGarrett must try to save Danno's reputation by proving that the robbery did occur and that the robber tricked the shooting victim into walking into the line of fire.

Operatives working for Wo Fat steal a device from a U.S. military base in Hawaii. Wo Fat is also manipulating a young Maoist into helping him smuggle the device to China. McGarrett and Five-O race to keep the device from leaving the islands. They capture Wo Fat at the last minute. But McGarrett receives a shock courtesy of U.S. spymaster Jonathan Kaye.

A military scientist is kidnapped by a co-worker and held ransom for one million dollars in diamonds. Five-O identifies unique background noises from the taped ransom messages to locate the kidnapper.

The slaying of a public health official while conducting an investigation into venereal disease leads McGarrett into the world of politics and intrigue. Starting with an appointment list of the slain official, McGarrett works his way up to a meeting with a highly respected candidate for public office.

To the horror of workers at a construction site, the truck of sand they are unloading contains Danny Williams in the back, unconscious and just moments away from having smothered to death. Danny awakens, but can't remember anything about how he got there. McGarrett grills the sand-truck driver, but eventually abandons him as a dead end. The only other way Danny could have gotten into the dump truck was to jump in from a height as the truck passed below him. Danny remembers he was going horseback riding on his day off, so the Five-O team heads out to a ranch that rents horses for pleasure rides. Eventually they find the body of the horse he was riding, shot multiple times and leaving a long trail of blood. That means Danny was fleeing from gunmen and made his leap as a last resort. But what did he see that made the gunmen chase him. The answer turns out to be a literal suicide bomber, the ranch owner, who is getting revenge on a Chinese visitor by having a bomb-laden boat (with mannequins at the helm, which is what Danny saw being loaded into a truck headed for the harbor) "steering" it by remote control straight into the man's yacht with the dignitaries aboard. (Presumably the thugs got paid in advance). McGarrett and the Five-O crew must race the clock to destroy the bombers' boat before it can complete the madman's mission.

Chris Vashon is male heir to the Vashon crime family, which now has considerable legitimate business holdings. The young Vashon, a rebellious sort, has been conducting robberies with friends. McGarrett sees the development as a way of striking back at the Vashons. However, Five-O discovers that's easier said than done, when Honare Vashon, Chris's father, bribes witnesses and Chris Vashon beats the rap in court. McGarrett doesn't give up and attempts to trap Chris Vashon again.

The Vashons are planning to go after Steve for killing Chris Vashon. His grandfather wants to send Steve a message that he's going to die. Honore, Chris' father, objects, unsuccessfully. Steve gets the message and goes to see Honore Vashon, who denies having anything to do with it. Chris's grandfather then hires someone to kill Steve but the attempt fails. Steve then has Honore placed under surveillance which the latter discovers and becomes very careful. He stops using his phone. Eventually he arranges a secluded meeting with his father who tells Honore they need to find someone outside the islands to take care of Steve. Eventually Honore evades the surveillance long enough to meet with the assassin. Steve knows there's someone out there with orders to kill him so he needs to find the assassin first rather than vice versa.

Dominick Vashon, the patriarch of the crime family, seeks revenge on McGarrett for killing his grandson, Chris, and sending his son, Honore, to prison. With help from his incarcerated son, he finds a man who has a grudge against McGarrett and asks him to kill McGarrett. He agrees and tries but McGarrett kills him first. When McGarrett goes to the body the gun the man had with him is gone and there are no signs of any bullets. John Manicote charges McGarrett with murder in the second degree. McGarrett hopes Harvey Drew, a lawyer who was with him, will testify to the shots that were fired. But he testifies that what he heard might not have been gun shots so Steve is indicted. Steve sees Dominick Vashon in the gallery and goes to talk to him. Afterward, McGarrett deduces Vashon sent the man to kill him. He needs to prove how the gun disappeared and why Harvey Drew lied.

A group of native Hawaiian separatists threaten to bomb public buildings if seven of their associates are not released from jail. McGarrett races the clock to find the bombs, and those responsible, before the deadline.

The Lovejoys, a family of grifters, make a bigger score than they ever imagined. The problem is they've ripped off a mobster who wants his money back -- and will gladly kill to do so.

A hippie couple kidnaps a baby from a drug store and smuggles the baby to the mainland to sell the child to an adoption agency in Los Angeles. McGarrett and the rest of the Five-O team are able to find that baby as well as another infant that was kidnapped a few months earlier. Now they must track down the young couple before they snatch another baby.

McGarrett is called in to talk to Toni, a tough-talking young woman who has recently been busted along with her boyfriend. Toni is going to prison and can live with that, but she wants to get married (she's pregnant) before then in an elaborate ceremony. In return, she will testify against a mobster who has always beaten the rap -- "How about murder one? ... With his own two hands?" McGarrett counters that the mobster will use all of his many resources to knock off the couple, but Toni is adamant and McGarrett goes to work with the help of the reporter he's dating.

A psychiatrist is tormented by a man calling himself Cerberus. Cerberus claims he was rejected for treatment and is determined to take revenge by extorting the psychiatrist. Cerberus, to show he means business, begins harassing the psychiatrist's patients, including a young man dying of a brain tumor and a suicidal woman. McGarrett & the Five-O must deal with this cunning and ruthless adversary.

The No. 2 executive to a reclusive businessman approaches McGarrett. He tells the lawman that he has evidence of illegal activity by his employer. But the executive insists on elaborate procedures for a more detailed meeting, including having McGarrett attend unarmed. The Governor insists that Danny Williams attend in McGarrett's place. While their meeting occurs, the executive also shows up and kills the chief executive of the company. Now, Danno is the executive's alibi.

Murdock, an embittered businessman, concocts an elaborate robbery to gain the millions of dollars he needs for a pet business project. Murdock, though, has underestimated the ruthlessness of the men he has hired -- two people are killed before the robbery even takes place. McGarrett is determined to crack the complicated case.

A man named Winkler goes berserk when approached by a television reporter doing a "man on the street" story. After his arrest, Five-O and HPD can find no evidence that Winkler even existed until seven years earlier. For Five-O, this is just the start of a complicated investigation full of shadowy figures and murky motives. Winkler, for example, has a number of aliases and a photographic memory. Eventually McGarrett & Co. uncover a plot to assassinate a Soviet defector.

Two men kidnap a young girl and hole up in a World War II bunker on Diamond Head. McGarrett and his men must rescue the child from the desperate criminals.

Jimmy O'Hara, co-owner of a travel agency with ex-bookie Sam Green, is beaten to death by thugs with brass knuckles. Green organizes gambling junkets, which is draining revenue from a hood's gambling rackets. What's more, one of Green's customers refuses to make good on $120,000 in losses on a junket in Seoul. Green is being squeezed hard by the Hawaiian gangster and the Korean gambling house. Five-O's investigation becomes complicated after the losing gambler turns up dead; the loser was also having an affair with the wife of another customer on the Korean gambling junket.

Five-O seeks to bust a family-run protection racket. But the investigation turns personal for Five-O stalwart Chin Ho Kelly because his daughter is in love with the son of the racket's patriarch. The probe intensifies after a policeman and the best friend of Chin Ho's daughter are killed. On top of that, the lovers wed.

A cat burglar robs a former CIA agent of a diamond pendant and a roll of microfilm. The film contains military secrets and soon agents from China and Russia, along with its original owner, begin vying for its possession.

Five-O and District Attorney John Manicote think they have an ironclad case against a mobster on trial. But the jury keeps coming back with news that they have not reached a verdict. The judge refuses to reveal the vote, but McGarrett correctly suspects that one person is holding out for an acquittal. But who - and why? Five-O obtains a jury list and starts checking out all of them to see who could have been bought, blackmailed or threatened.