Assistant Prosecuting Attorney Charlie Cadison arrives at his office on a Sunday to pick up some paperwork relating to the upcoming trial of local racketeer Joe Matsukino. When Cadison enters his office, he finds a boxman (safecracker) who is snooping and has found a gun which is an important piece of evidence in the trial. The boxman pulls out his own gun and shoots Cadison dead. Cadison's wife Helen, who was waiting for him in their car outside, and who is massively pregnant, rushes into the building to find her husband's body on the floor. She is distraught.
Five-O is on the scene. A guy named Pete, presumably from the prosecutor's office, tells McGarrett that among "everything" that was stolen, the gun was "the one solid piece of evidence that nailed Matsukino cold." Danno tells McGarrett says the boxman was "no amateur," probably someone from the mainland. As McGarrett comforts Helen, she starts to have contractions. McGarrett gets Chin to call an ambulance so she can get to the hospital right away.
Matsukino and his right-hand man Harry Cardonus (Nehemiah Persoff) are sitting outdoors by what looks like a hotel pool. After they find out about Cadison's assassination, Cardonus roars with laughter, anticipating what will happen when the judge at the trial in two days asks "What are you gonna use for evidence?" Danno and Kono show up, telling the two men "we've got a few things we'd like to talk over with you at the Five-O offices." When Matsukino and Cardonus balk, saying what are the charges, Danno says "How's murder grab you?"
In his office, McGarrett tells Matsukino "Ten years you've been trying to put this island in your pocket. But when you killed Charlie Cadison, you blew it." When Matsukino says "I gotta give a little speech to the businessman's association Tuesday," McGarrett tells him "Tuesday afternoon, you'll be on your way to Oahu State Prison for life." He continues: "We're gonna nail you for killing that bookie, with or without the gun. That's murder number one. But if that doesn't work, I'll spend every resource of this office to convict you for the brutal murder of my friend, Charlie Cadison. That much I promise you. If it's the last thing I do at this desk, I'll see you in Oahu State Prison for life."
Matsukino is escorted out, but McGarrett tells Cardonus to sit down. He plants a bug in Cardonus' ear, telling him that "Matsukino, king of the rackets, is running scared … When a man runs scared, he'll get rid of any possible threat. You're a threat to him, Cardonus. You're the only man alive who knows as much about him as he does himself."
When Cardonus returns to poolside at the hotel, Matsukino is curious about what transpired after he left the Five-O office. Cardonus tells him, "He didn't turn up nothing." Matsukino leaves, giving Cardonus a peculiar look, and when Holly Flick, Cardonus' 18-year-old girl friend, wants to go swimming, but has left her bathing suit at home, Cardonus tells her to go get it, giving her his car keys. When Holly starts the car, it explodes, killing her.
At Five-O, McGarrett is in contact with the hospital. It is touch and go as to whether Helen Cadison is going to lose her baby. Danno reports there are no clues as to who was behind the bombing of Cardonus' car. McGarrett tells Danno to find the boxman, since there are only 24 hours before the trial the next day.
Cardonus has disappeared, but he shows up at the hotel, where Matsukino acts as if he doesn't know anything about the bombing. Cardonus is incredulous that Matsukino, who has been his "pal" and "buddy" for "over 10 years" is now trying to knock him off. Cardonus says "I'm gonna blow the whistle on you!" Matsukino tells him "You're making a mistake."
Cardonus goes to the Five-O offices, where he tells McGarrett he will give him "the whole works," including names and addresses that he kept as Matsukino's bookkeeper, that he wants to "lay it on Matsukino so hard that he'll fry in hell." McGarrett offers Cardonus "no deals," saying his deposition is not enough. He wants Cardonus to testify in court at the trial the next day. Cardonus describes himself as a "walking dead man," but McGarrett offers him protection. Cardonus scoffs at this offer. McGarrett makes Cardonus a bet that they can keep him alive, and Cardonus replies, "You're on."
Cardonus is taken to the Kahala Hilton. Despite "a camera covering every entrance and exit on this floor" and "everybody else … cleared out of this wing of the hotel," a gunman named Jerry Apu is encountered in Cardonus room who takes a shot before McGarrett kills him.
Chin Ho investigates, and finds that Apu descended from a floor above using a rope. McGarrett tells Chin to make sure this wing of the hotel is buttoned up tight, "I mean tight." Cardonus tells McGarrett "You talk like a Boy Scout. You ain't got a chance. I'll be long dead by morning."
On the beach, Danno and Kono talk to Oscar (Randall Lo), a local boxman and pickpocket who might know something about the man who killed Cadison. Oscar takes them to a house and points out where a man from the mainland was seen, according to "some talk along the street." When Danno and Kono go inside this joint, they find fixings, suggesting the man is dope addict.
Cardonus is ensconced in his hotel room, but he keeps ridiculing McGarrett about Five-O's efforts to keep him safe. He tells McGarrett that he was the one who arranged for the boxman to come to knock off Cadison. This just infuriates the Five-O boss, but gives him even more reason to keep Cardonus alive. Cardonus says that his situation is "pretty funny"; McGarrett tells him to shut up.
Cardonus provides a description of the boxman, whose name is Murphy, and McGarrett gets this information relayed to Danno. Kono and Danno locate Murphy, who is looking to score some drugs. He flees and shoots back, but some of Matsukino's men are also after Murphy. They shoot him. Murphy tells Danno he dumped the gun used to shoot Cadison in the harbor and provides some information about being hired by Cardonus, but he dies, making everything he said useless.
Back at the hotel room, Cardonus treats his captors like room service. When a cop goes to get him some water to mix with his drink of Scotch, the cop has a drink of water himself, and drops dead because the water supply has been tampered with – it sounds like it contained cyanide.
Cardonus is complaining about his food. McGarrett takes away a cigar that the man with the food brought and gives him a package of gum instead. Shortly after this, Cardonus collapses to the floor, suffering from some unspecified condition. A doctor who is summoned says Cardonus is suffering from "a severe allergy attack" and he is rushed to the hospital.
At the hospital, Helen Cardison has given birth to a baby boy. Things don't look good for Cardonus, on the other hand, about whom a doctor says "He is not breathing."
The newspapers print a story with a huge headline: "Trial witness dies." At the courthouse the next day, Matsukino shows up and gloats at McGarrett as he sees this headline. When an ambulance pulls out at the courthouse, Matsukino wonders what this means. McGarrett says, "Has to be some way to get a star witness here." When Matsukino says that "Cardonus split the scene last night," McGarrett says that Matsukino should have read the newspaper story more carefully. The "secret witness" with "key testimony" was Murphy, the boxman who Matsukino ordered killed. Cardonus is taken out of the ambulance and brought towards the courtroom on a stretcher. When Matsukino tells Cardonus "You owe me, Harry. You owe me," Cardonus replies, "The only thing I owe you for is a good night's sleep in the hospital. No disturbances, no attempted hits on me." Cardonus is taken into the courthouse as McGarrett reiterates what he said earlier: "I promised you, Matsukino. Oahu State Prison, life."