As Steve McGarrett jogs along the beach in some out-of-the-way location, a guy drives up to the place where McGarrett is parked. Nearby, McGarrett stops for a moment to help some little kid who has wandered into the surf. The kid's mother, Mrs. McGovern (Anna Lea) is grateful. McGarrett continues jogging, and when he gets back near his car, he thinks he knows this guy, who has been watching him approaching from behind the bushes. The guy shoots him three times and then flees from the scene.
Alerted by a phone call from Mrs. McGovern, cops and an ambulance arrive to find McGarrett's body. Danno takes charge of the situation. He later goes to see the Governor and the Attorney General. The Governor, while telling Danno to "get him," emphasizes that just because McGarrett is out of action, Five-O's responsibility to the people of Hawaii doesn't stop.
There are no clues at the scene of the shooting which will help Five-O to determine who was responsible for the hit. Danno guesses that it might be Charley Mangan (Paul Picerni), who McGarrett was after recently as part of a narcotics smuggling case.
Danno and Kono track Mangan to a yacht harbor. Mangan is roughed up by Danno. Mangan says although he had a contract out on McGarrett, he cancelled it because McGarrett had a talk with him recently and said, "I understand you're looking to have me hit. Well, if you find somebody, you better make sure he does a good job." Mangan is arrested. When asked what for, Danno says "loitering."
Around this time, a carpenter named Carl Swenson is shot dead in Pearl City.
Kono reports that Mangan's alibi for the time of McGarrett's shooting is confirmed by seven witnesses.
McGarrett's condition is described as "extremely critical." He was shot once in his arm, grazed in his side and had a direct hit in the abdomen.
On a hunch, HPD Lieutenant George Lealoha (Doug Mossman) had the slugs from Swenson's shooting compared to those of McGarrett's, and they came from the same gun. Swenson's landlady Miss Feathertree (Ellen Corby) tells Danno that she is shocked that Swenson, who has lived at her place for 10 years and she describes as "a sweet soul," was killed.
At the hospital, McGarrett is rushed back into surgery on the orders of Dr. Cohen (Al Eben).
The guy who shot McGarrett goes to visit his wife.
At this point, we still do not know who this guy is, but we later find out he is Joseph Trinian (John Larch), who has been in the Portsmouth, Maine military prison for the last 15 years because of a homicide and other charges. He is seeking revenge on those people who put him there, including McGarrett, who was the naval intelligence investigator in Korea who broke the case open as well as Swenson, whose real name is Rudkers. Rudkers was the chief witness against Trinian.
Trinian's wife Emma (Vivi Janiss) changed her name to Crawford after her husband was sent away to jail, but she still wears his wedding band. Now she is an office manager in a small Honolulu insurance company. Trinian tells his wife he wants things to be the way they were. When Emma tells him she doesn't know if she can leave her "different life," he tells her, "I've learned to be patient waiting for what I want."
Dr. Cohen tells Danno that McGarrett's chances are seventy to thirty -- against. At the office, Danno is busy taking care of cases, hearings and meetings McGarrett would have been dealing with.
Chin Ho goes and talks to some local hot-rodding punks. When he asks their leader M.K. (Lanikai) who would like to see McGarrett dead, he is told "Everybody and nobody." Chin wants to know anything M.K. hears, saying "If we learn you held out any information, you're gonna have more heat than you ever thought possible."
Five-O finally gets a break. A fingerprint report on Swenson back from Washington reveals he is Rudkers, and a connection between McGarrett and Trinian is made, along with the fact that Trinian got out of jail three weeks ago.
According to reports from the Navy and the FBI, Trinian was a chief petty officer in supplies, stationed in Seoul who Danno says "had his dirty fingers in a lot of different things". A young sailor was found dead in an alley, a big winner at a gambling joint owned by Trinian and a Korean. McGarrett dug up Rudkers, a witness who was involved in the gambling, who connected Trinian to the murder. Trinian threatened McGarrett after he was convicted, claiming he was framed.
Emma tells Joe how that after he was sent away, she was ostracized by all of her friends. Trinian explains the reason he has come to Hawaii is "to get rid of those years and the things that caused them … to wipe them out completely." He says, "I'm going back to the mainland tonight, just as soon as I finish a little business I have here. And I'd like you to go back with me."
Emma doesn't know if she can make this big adjustment, telling him, "You're asking too much too soon, Joe." He tells her to "think about it" and he will be back in a little while after he takes care of the "business" he mentioned.
After Danno gets a call from Naval Intelligence that Emma Trinian is living in Hawaii, he goes to visit her. She is evasive and hostile when he starts asking her questions about her husband. She knows McGarrett is "the Naval officer who persecuted Joseph in Korea some 15 years ago," and that McGarrett "brought in a false witness and manufactured evidence." Danno trips her up, making her admit that she has seen her husband recently, telling her that "you've talked it over with him and you believed everything he said." She refuses to say anything else, adding "if you want to arrest me and throw me in jail, you go right ahead." Danno leaves, but tells some HPD cops to keep the house under surveillance.
Trinian phones his wife shortly after this, and she tells him Five-O has been looking for him. She says, "Why did you lie to me, Joe? Why didn't you tell me why you'd come back here?" He tells her, "I was so careful. I covered my trail all the way. They shouldn't have found out it was me ... From the first day, all I could think about was getting McGarrett. He crucified me, Emma, so I shot him." He adds, "There's still time ... I can finish it ... I can still do it." Emma implores her husband to give himself up, telling him he needs help, but he just hangs up the phone.
Emma phones the Five-O office and wants to speak to Danno, but he is not there. Her message that her husband is going to kill again is relayed to Danno who quickly goes to her house and tries to figure out who the next victim will be. Through a process of elimination, she brings up the name of Commander Stewart, the Naval Intelligence prosecutor at her husband's trial, who, it turns out, is Walter Stewart, the Attorney General of Hawaii!
Danno and Emma quickly drive downtown where Stewart and the Governor are observing the King Kamehameha parade which is in progress. Trinian, who has obtained a naval officer's uniform from a tailor shop, enters the Iolani Palace and goes up to the balcony to where Stewart and the Governor are standing. He pulls out his gun and is about to fire just as Danno and Emma rush into the building. Danno yells at Trinian, but getting no response, pulls out his gun and shoots Trinian dead.
Emma kneels over her husband's body, saying "He's left me, Mr. Williams. He's left me again."
Soon after this, Danno meets in the hospital with McGarrett, who will recover from his wounds. Charley Mangan will be indicted on drug-related charges, thanks to a tip which M.K. provided to Chin Ho. Danno tells McGarrett that his job is "a heavy load." McGarrett tells Danno, "Well done."