Harry K. Brown, real name John Warnesh (Edward L. Drew), one of the world's top safecrackers who has never made a hit under a half-million dollars, disembarks from a plane at Honolulu Airport with a babe on his arm. At the United Airlines information desk, he is given a key to a locker which contains a briefcase with a hotel reservation and $20,000 cash. Danno and Chin Ho are standing around looking very stunned. After he opens the locker, Chin approaches and pulls out his badge, saying Brown is under arrest. Chin is kicked to the floor by Brown, who whips out a gun. Danno pulls out his gun and shoots Brown dead.
At the Five-O office, McGarrett tells his men their assignment is to figure out what Warnash was doing in Hawaii. McGarrett says he "cracked a few safes for naval intelligence" which presumably qualifies him to be an expert in this field. They quickly head to the Big Island where McGarrett, assuming Brown's identity, checks in to the Mauna Loa Hotel where Brown had his reservation. There are several babes hanging around the hotel. McGarrett asks the desk clerk, "How's the action around this place?"
After entering his hotel room, McGarrett finds a listening device in the chandelier, but only seconds later, Carl Swanson (Gerald S. O'Loughlin) shows up with a gun, wanting to see Brown's identifying tattoo on his arm which McGarrett has fortunately duplicated, hopefully not for real, and wanting to know why Brown is late for his assignment. Swanson says "The Man is paying you top dollar. That means you show up when he says and where he says."
Turning the tables, McGarrett takes Swanson's gun away, saying "The Man wants me checked, he checks himself." Swanson tells him, "This caper is a one-way contact only. I don't know where The Man is or who he is or what he is."
The two of them take a cab to an expensive-looking oceanfront house where McGarrett meets the other two members of their team, the Frenchman Andre Maurac (Than Wyenn), whose specialties are "the torch, X-ray and saw," and Margi Carstairs (Antoinette Bower), who is "attached to the entourage of a diplomatic emissary by the name of Quan Ling," who has diplomatic immunity. Swanson is said to be "an expert electrician."
They receive instructions from The Man via reel-to-reel tapes as well as phone calls and letters. The Man has already heard what happened in Brown's hotel room via the bug, and he cautions Brown not to step out of line any more. Brown flirts with Margi, saying she has "class," but she tells him "I can see that you talk too much." On the tape, The Man tells Brown, "I'm paying for a box man, not a lover." They still do not know what their assignment will be, only that it involves "a guaranteed payoff of $1 million, split four ways."
Using a long-range telephoto lens, Danno takes pictures of McGarrett and his associates from offshore at their house while pretending to be on a fishing expedition.
Danno returns to shore and then to Honolulu to update the Governor on what is happening. He tells the Governor their investigation reveals the house was rented a month ago by one Hiro Tagati from Tokyo and it was paid for six months in advance, all cash. Some pipe-smoking guy named Frank Wayne (Robert Errecarte) also in the Governor's office with Danno, ostensibly a federal agent, says "So far, our Tokyo agents have been unable to locate anyone operating under that name."
Back on the Big Island, McGarrett snoops in an envelope which has arrived from The Man, which causes Swanson to accuse McGarrett of being a spy. This envelope contains a blueprint of the ship where they have to carry out their assignment. It is called the Antiya and is owned by Quan Ling. From a tape in the envelope, The Man tells them, "Memorize every passage, every foot of her, fore and aft." They are ordered to "make the necessary contacts and the necessary purchases. You have already been informed what will be required in the operation."
After they go to the docks to check out Quan Ling's yacht, described as "the size of a coast guard cutter," which is floating nearby, McGarrett gets very chummy with Margi. He gives her a kiss and buys her a lei, saying "From Harry K., with love." He then takes a cab driven by Kono to a tennis court where he talks to Danno and brings him to date about the caper. McGarrett then goes back to the Mauna Loa Hotel where he is supposed to meet some guy (Ken Hiller)who will give him some nitroglycerin to help blow the safe on the yacht. Just as he is getting the stuff, Brad Warren (Milton Hibbon), a hotel security guard, breaks McGarrett's cover, and the guy with the nitro pulls a gun.
McGarrett threatens to detonate the nitro, so the delivery man tries to run away, but Kono, who is nearby, trips him up and the guy is busted.
Back at the house, McGarrett shares a drink with Margi, who asks whether she can trust him. She gets philosophical, talking about how ordinary people can become monsters: " I suppose there's just a little girl in all of us, just looking out, wondering why people turn into such frightening, misshapen monsters. And then one day, you look and you're one of those people. You're one of those frightening, misshapen monsters. Crowding, destroying, doing anything to stay alive." McGarrett says, "Come on, baby, we were talking moonlight and orchids, remember?"
The job will be pulled off that evening, and the foursome have only eight minutes to complete their task. There are three cylinders in a safe, weighing 2 kilograms each, and the safe has to be detonated without destroying them. Considering the price of gold is a mere $35 an ounce, McGarrett speculates that these cylinders contain six kilos of pure heroin, worth $40 million.
Later, during a party on the yacht, Swanson goes aboard posing as a refrigerator repairman working for theMauna Loa Refrigeration Service, while McGarrett and "Frenchy" Maurac approach it by swimming (no one sees them). They punch out a couple of guards and make their way to Quan Ling's room where he is passed out from a Mickey Finn that Margi slipped him. Using a metal detector to determine its location, Maurac cuts a hole in the wall and McGarrett places plastic explosive on the safe's door. Swanson, who has gone to the electrical room, is supposed to cut the power at the moment the nitro goes off to create confusion, but he is interrupted by a guard, and during the ensuing fight, Swanson's synchronized watch is broken. He just guesses at when to cut the power, which is the wrong time for the others, but the safe manages to get opened, they grab the heroin and quickly get off the yacht with no one the wiser. (Margi comes wearing her bathing suit.)
Having returned to the house, the team expects to get paid for their work. A tape from The Man tells them to remove the top from a nearby stone lantern (known in Japanese as an ishi-doro) which will contain their payment, and they are to put the heroin inside. But when they open it up, they find it empty. Margi pulls out a gun and shoots both Maurac and Swanson. She is about to drill McGarrett when he points out that Danno, Kono and some other cop are on the roof nearby pointing guns at her. McGarrett plays back the tape, adjusting the speed, which shows us that The Man was actually Margi. McGarrett tells Chin to "book her."