Danno pursues 18-year-old Thad Vaughan on foot through the streets of Honolulu. Vaughan (James Lloyd Land) fires a shot at Danno, then goes into his fleabag apartment building. Danno follows him as other residents of the place run out. When Thad will not open his door, Danno fires a shot through the lock, which fatally wounds Vaughan. Danno kicks the door in. Vaughan says a few words, then expires. Unseen by Danno, Thad's girl friend Ann (Charlotte Considine) picks up Vaughan's gun and quietly leaves the building.
McGarrett and HPD cops are on the scene investigating the shooting, along with Nat Schneider (Jeff Kennedy), who seems to be connected with HPD. This is the first time that Danno has killed someone as a cop, though he was off duty at the time, and we later find out he had a couple of beers. Danno doesn't understand how he shot Vaughan through the door. He saw Vaughan trying to break into a car earlier and pursued him with the intention of making an arrest.
The apartment has signs of drug use -- a marijuana roach -- as well as evidence of someone else living there, quite likely a young blonde woman. The cops cannot find the gun anywhere.
On the beach near the Ilikai Hotel, Ann is distraught.
McGarrett tries to get Danno to remember what happened, saying "A cop pulls his gun, he better remember every single detail. There's always trouble when you pull a gun, more if you fire it. You hit somebody, and you're up to your hip pockets in it. And if a cop kills somebody, every single fact better jibe or he gets nailed to the wall. He's guilty until he proves himself innocent. Now, that's backwards, ugly and unfair, but that's the way it is. Now, we gotta know everything there is to know right now."
Even when the two of them retrace Danno's steps at the parking lot where Vaughan was trying to break into the car and back at the apartment, Danno's mind is a blank. But McGarrett demonstrates to Danno that someone could have taken Thad's gun from the floor when he was not looking after the shooting.
Ann is still on the beach. When some drunk approaches her, trying to pick her up, she threatens to shoot him with the gun. The guy leaves.
The next morning, Che Fong (Edward Tom) digs a bullet out of a wall near Vaughan's building.
Ann goes to visit Big Chicken (Gavin McLeod), her pusher. Ann wants some dope, but Chicken won't give it to her, since she has no money. After she pleads with him, he says he will stake her out for a couple of days until he gets back from a trip to Maui. Chicken tells her that she needs to find another boyfriend.
At the Five-O office, McGarrett and Danno watch Fred Vox (Joe Rose) deliver a commentary on TV. Vox says that unlike an average citizen under similar circumstances, Danno, who was drunk when he shot Vaughan and cannot identify the car Vaughan was breaking into or produce the gun he was using, is walking the streets, "still loose with his license to kill."
Walter Stewart, the Attorney General (Morgan White), calls McGarrett, saying that Vox is coming to interview him soon and it would really help if they can locate Vaughan's gun. As he leaves, Danno tells McGarrett "It's a stinking job." McGarrett replies, "Who told you it was anything else?"
McGarrett sends Chin and Kono out to try and find witnesses to what happened. Kono talks to Frieda (Kukhie Kuns), who works in a massage parlor, suggesting she make up a story to back up Danno, but she declines. Chin interviews a Mr. Wang (Arthur Kee), who Vaughan bumped into when he was running away from Danno, but Vaughan didn't have his gun out at the time.
Schneider shows up at the Five-O office to tell McGarrett the grand jury indicted Danno on a charge of first-degree murder. Danno goes to police headquarters to turn himself in, including being fingerprinted.
McGarrett is outraged by the situation with Danno. He goes to see the Attorney General, who tells him to "sit down and cool off." Stewart tells McGarrett that with no evidence of a gun and information about the car that Vaughan was breaking into, and with only Danno's word as defense, he will "go to jail for the rest of his life."
As McGarrett is leaving in a huff, Stewart tells him, "You know he's not guilty, I know he's not guilty. But if I were Danny's attorney, I'd want him right where he is, in a cell. And I'd do everything possible to delay that preliminary hearing until you find that gun and develop that chain of evidence which puts the gun in the room with the boy at the time he was shot. Now, you do that, and Danny's off the hook. Steve, there's only one person who could do anything about it. You."
McGarrett returns to his office where Kono tells him that HPD had 37 sets of Vaughan's prints taken from cars he was breaking into, plus a store name from Philadelphia in his shoes. Big Chicken's fingerprints were also found in Vaughan's room. McGarrett speculates that while Thad had no needle marks on him, it is quite likely that the "wahine" living in the room with him is a dope addict. In fact, Kono says "With Big Chicken in the game, ain't much doubt somebody is, but it wasn't the boy."
McGarrett goes to the Hubba Hubba Club where he talks to Bonita (Ann McCormack), one of the strippers, who is a friend of Chicken's. She tells him where Chicken hangs out.
Kono goes to the jail to deliver some Chinese food to Danno. While he is there, Schneider drops by to say they found Thad's gun. Che Fong does a comparison test, and it is the same one that fired the bullet they recovered from near Vaughan's place. But there is no link between it and Thad.
Schneider got the gun via a low level criminal named Al Drucker who obtained it from some punk named Tommy Tommy (Alan Naluai). McGarrett and Kono go to see Tommy Tommy on the beach where he is hanging out with his gang and peddling a stolen car stereo. After he calls McGarrett "Mr. Fuzz" and gives him some mouth, McGarrett and Kono seriously rough up Tommy Tommy and some of his gang, and McGarrett convinces him to admit that he got the gun for three dollars from "a blond chick" named Ann who was a junkie.
When McGarrett returns to his office, he finds Big Chicken waiting for him there.
Chicken doesn't want to admit that he knows Ann, because he is a three time loser and associating with her could put him back in jail. Aaying "I'm gonna tell you because the law is cool," he gives McGarrett a favor "as a good citizen," that Ann is at the "little jungle, Maggie's pad." McGarrett finds Chicken loathsome, telling him "I'm gonna nail you." Chicken leaves, saying "Never, Mr. McGarrett, no chance you'll ever. No way. Peace."
McGarrett goes to Maggie's, where Ann hides in the attic and he has to deal with two wacked-out hippie types. Maggie persuades Ann to surrender. Ann is taken to the hospital to dry out, where she admits that she witnessed the shooting of Thad and then she took the gun and ran away with it. Later she sold it because she needed the money.
Ann says she made Chad steal, and Big Chicken taught him what to do: "Chicken would tell me what he wanted Thad to steal and who to deliver it to. Thad would give me the money, I'd give it to Chicken. He'd tell me where to pick up my stuff. Once Thad got going good, Chicken never wanted to see him at all. Thad only took what Chicken wanted him to. Cameras. Parts for cars. Now he's dead." To reward him, Chicken gave Thad a surfboard which the cops saw earlier at Vaughan's place. McGarrett tells her that Big Chicken killed Thad, not her. "He sank the hook in you and he killed Thad."
McGarrett goes to Chicken's place and roughs him up. Chicken protests his innocence, saying there is nothing in his place that could tie him to any crime. But McGarrett says there is a stolen carburetor in the refrigerator at Vaughan's place and Ann will testify that he ordered it stolen. They also talked to the man that Thad was supposed to deliver the carburetor to who swore that he ordered it from Chicken. McGarrett says "It ain't no big thing, brother."
McGarrett goes to the jail and gets Danno released. Danno says "I'm glad that's over."