Someone wearing rubber gloves arrives at a woman's apartment, as if for a date. She lets them in, saying she will be ready in five minutes. The person turns off the lights, then pulls out a knife and stabs her to death, leaving a card with a number "1" on it on top of her body. Then the person mails a letter.
In the next scene, this letter arrives at the Five-O office. A smaller envelope is in an outer envelope. On the flap of the smaller envelope, there is typing: "One for the money, Two for the show, Three to make ready, And four to go. Might even make it A hundred or so. This smaller envelope contains a picture of the murdered woman with a red "X" across her face.
McGarrett phones HPD Homicide and speaks to Lt. Palek (Bob Basso), inquiring if there have been any homicides since the previous evening. He is told that a woman answering the description of the photo was found dead.
McGarrett and Danno leave the office for the crime scene at Leahi and Monsarrat.
At the woman's apartment, she has been identified as Aimee Cross, single, 33, who worked as a secretary for Hawaiian Amalgamated Industries. McGarrett finds a photo of a company get-together from which the picture he received in the mail was taken. He tells Danno to talk to the photographer who took the picture as well as her company.
Another woman is murdered in her garage as she arrives home. She is also stabbed to death.
Kono reports that the photo received was a blowup from the large picture at Cross's place. The envelopes the killer used for mailing are not traceable. On the typewriter used to type the poem, the M and the A are chipped, with letters out of line. It is likely a portable, maybe 25, 30 years old. Danno managed to track down the photographer of the large photo. The picture was of a luau for the home-office staff at Cross's company
Another envelope is received with the same M.O. as before; an inner envelope contains a photo of a woman with a red "X" on her face.
With information from Hawaiian Amalgamated Industries, McGarrett, Danno and Pelak go to the latest victim's house where they find her body in the garage. She has a card with a number "2" on top of her.
McGarrett goes to the company where he talks to Charlie Gordlow (Paul Collins), the personnel manager, who is shocked by the two murders. The second woman, Mrs. Warden, had worked for them for almost 20 years. McGarrett tells Charlie he wants someone from Five-O to go through the company's records and also to question the employees.
McGarrett meets Arnold Martin (Farley Granger), the company's executive vice-president. He says that McGarrett's request will have to go through Aunt Martha, who owns the place. Arnold and Charlie are her nephews. Charlie says that they are " prime examples of the rewards and perils of nepotism."
Soon enough, McGarrett meets Aunt Martha, who says that it is "preposterous" that the murders have anything to do with the company. McGarrett tells her, "My job is to find out if it has." McGarrett says he wants to see the company's typewriters to compare to the message he received, but Martha tells him that they replaced all of their machines six months before.
Martha tells Arnold that he must co-operate with McGarrett and Charlie that she wants to see him at her home that evening.
McGarrett orders protection for the 70 people who attended the company luau, despite the fact that logistically this will be very difficult.
Charlie goes to see Aunt Martha that evening. She reminisces about when she gave Arnold a present on his 21st birthday, which was a typewriter. Charlie remembers this, and that she gave him "practical gifts" consisting of clothing when he had a similar birthday.
Martha asks Charlie if he recognized the typing on the letter McGarrett showed them earlier. Charlie knows it was typed on Arnold's portable, and that he typed the message, which Martha also says is "preposterous."
Charlie tells his aunt, "I've become impatient. And tired. I'm tired of you. I'm sick of living on your charity, being treated like a child. 'Don't do this, don't do that, do this, do that.' I'm a man, Aunt Martha. A man. And you'd never let me be one. You never let me do anything by myself. I could never enjoy life doing what you told me, living on what you paid me. But I can on half of what you're gonna leave. And I'll be free of you." He pulls out his knife and stabs her to death. He leaves a card with a number "4" on her body.
Using a copy of Gray's Anatomy, Charlie stabs himself in his stomach in an area where he avoids damaging any vital organs. After he phones the police, McGarrett quickly arrives, but Charlie cannot provide any definitive information as to who tried to kill him. He is rushed to the hospital.
McGarrett gpes to Martha's place, only to find her dead. At the hospital, McGarrett talks to Charlie who describes how he was stabbed by someone whose voice he almost recognized. Arnold shows up at the hospital room holding a cassette recorder with instructions to play a tape inside. Aunt Martha left this with instructions they were to listen to the tape after she died. In what is like a living will, Martha leaves the controlling stock and the management of the company to Arnold, and leaves Charlie income from a trust, "enough to live very well, but not too wildly."
Chin Ho reports that the typewriter was a Star-Gaymont, manufactured in 1948. Kono brings some photos that Che Fong took of Charlie's apartment. McGarrett tells him, "Don't ask me what or where, but somewhere in these photographs must be the answer."
Charlie is seen driving on a narrow road in the middle of nowhere above some high cliffs. He is thinking about what his aunt said about him on the "living will" tape. Charlie goes to Arnold's place at 6 a.m. to return Arnold's typewriter which he borrowed, and to talk. Charlie soaks some handkerchief in a substance like chloroform and knocks Arnold out with this. He then then takes Arnold's body and puts it in the garage with the doors closed and his car running.
At Five-O, McGarrett has a hunch that Arnold is behind the murders, because he stood to benefit the most from his aunt's death. He and Danno go to Arnold's place. When they get there, the find the garage door closed with the car running inside. They cannot open the door, so McGarrett gets the police dispatch to generate a high-pitched noise which unlocks the door via McGarrett's car radio. An ambulance is requested to Arnold's place, and the attendant says that he will survive. Arnold is taken to the hospital.
In Arnold's house, Danno finds the typewriter which he figures was used to create the poem on the envelopes along with rubber gloves and a knife in the car's glove compartment. But McGarrett's not buying any of this. He says the killer had to be "someone without conscience or feeling of guilt." McGarrett returns to Charlie's apartment because "something about this door bothers me." He quickly figures out what is fishy. There are "no bloodstains at the door. He said he was stabbed at the door, he didn't bleed at the door. Bloodstains at the desk. All the way to the phone, but there are no stains at the door, or from the door to the phone."
McGarrett and Danno pay a visit to Amalgamated Industries, where Charlie is playing golf out on the front lawn. They tell him that Arnold is going to recover in the hospital, and when he does, he will likely tell them who tried to kill him. Charlie goes to the hospital ostensibly to see his cousin, but intending to knock him off.
Charlie talks to Arnie, saying "You know, the way I figure it, dying is dull. But I guess you're gonna have to put up with being killed again. I'll tell you one thing, it wasn't my fault. Way I had it planned, you should have died in the garage. McGarrett, he worries me. Sometimes I think he knows something. But whatever he knows, he won't be able to prove it. Because he needs you to prove it, and you won't be around."
Charlie hallucinates, thinking his aunt is the one in bed instead of Arnold. He tells her, "If you don't go away right now, I'm gonna kill you again." He attacks the bed with his knife, but it is not Arnold in the bed either, but Danno, who quickly jumps out. McGarrett rushes into the room as Charlie starts stabbing the walls, saying he had to kill his aunt because she hated him. He thinks McGarrett is his aunt, and surrenders the knife to him, saying "You all look at me like I'm crazy. I'm not." He is taken away.