At the Flynn Armored Car Company, Harry Kegel (Chuck Couch) quits his job and is wished well by his fellow employees. Later outside his apartment, Kegel is about to get in his car with a suitcase of money when he is hailed by Sakai (Seth Sakai) and Vic Salazar (Emilio Delgado). He jumps in his car and speeds away pursued by the two men. In desperation he drives up the exit ramp to the freeway, his car flips over and catches on fire, killing him and burning up most of the money.
McGarrett arrives and learns that the serial numbers for the remaining bills match money stolen in an armored car heist 18 months before. McGarrett says that Kegel's death puts a big dent in Five-O's investigation. He wants to know the answers to questions about Kegel.
In San Francisco, ex-HPD cop Marty Cobb (Gil Girard) learns about the car crash and Kegel's death on the local news. He returns to Hawaii shortly after and visits a bar run by Eddie (George Herman) and inquires as to the whereabouts of the bar's former cashier, Mavis Tracy (Lynne Ellen Hollinger). Eddie says he hasn't seen her for about a year. When Eddie offers Marty a drink, Marty says he hasn't touched the stuff for the last 8 months.
After Marty leaves, Eddie visits Sakai and Salazar, telling them that Marty is back in town, looking for Mavis. Sakai is frustrated with the lack of action in Honolulu.
Marty visits McGarrett. He says he wouldn't blame McGarrett if the Five-O chief didn't want to speak to him. McGarrett's feelings haven't changed over Cobb's dereliction of duty during the robbery 18 months before -- because Cobb was drunk, the driver of the armored car was killed. Marty says when he heard what happened with Kegel, he decided it was time to apologize. He says that he has been attending Alcoholics Anonymous meetings where they have a program of twelve steps -- the ninth step says that you should make amends to all the people you harmed through your drinking. He repays McGarrett fifty dollars that he used to buy booze, but McGarrett returns it, telling Marty to repay him when he gets a job. Marty asks McGarrett to help him get information on Mavis's whereabouts, saying that he "messed up her life."
Duke and Chin check out Kegel's room, where they find a safe behind a couch.
McGarrett meets Marty at an AA meeting and gives him information about Mavis. He meets Marty's "sponsor," a woman named Gloria (Linda Redfearn), who says that AA members come to Hawaii on vacation too. McGarrett says that Mavis is a model in a dress shop downtown. After McGarrett leaves, Marty gets in his rented car. He takes a gun out of the glove compartment.
Mavis is shopping downtown. When she sees Marty approaching, she runs away, but he catches up with her. He says it took him a year to figure out that she put a mickey finn in his drink, knocking him out for 36 hours while the armored car robbery was taking place. He says that he ran away because he couldn't face her, and that he loved her. She says she spiked his drink because "they would have killed you." He demands to know who set him up. She finally confesses it was Salazar.
Danno talks to the maintenance chief at the armored car company (Wallace Landford) who says that Kegel was a dependable employee. He's surprised that Kegel was connected with the robbery. Danno suggests that Kegel rigged the truck so that carbon monoxide came in through the air conditioning system, killing the driver. The maintenance boss says that Kegel wasn't working on the trucks, but that he had access to them.
Salazar and Sakai scout outside the company, saying it's too difficult to break in and rig another truck. Sakai says it's like "looking for a weak spot in Fort Knox," and wants to try a bank job instead. But Salazar spots a room for rent across the street from Flynn's, and the two of them check it out.
At the Pearl City police station, Marty meets Barney, his former partner. Barney says that "alcohol is a disease" and that he should have seen Marty's troubles coming. Barney says his problems are no one's fault but his own. He enlists Barney's help in locating Salazar through the Department of Motor Vehicles.
Cobb tracks down Salazar to a hotel where he once stayed. The desk clerk tells him to check out a flower shop where Salazar's girlfriend Melia (Elissa Dulce Hoopai) works. Marty pays her to tell him that Salazar hangs out a bar called The Blue Mirror. After Marty leaves, she phones Salazar.
Five-O's investigation shows that Kegel lived like a hermit, and that he fooled everyone at Flynn's. He got a mysterious phone call the day he quit. McGarrett wonders if Kegel's fatal accident wasn't an accident, that the crooks connected with the armored car job 18 months before were involved. He says that Flynn's should check any car that goes out in the next while for tampering.
At The Blue Mirror, Marty goes into a back room and confronts a man he thinks is Salazar. But the real Salazar comes up behind him and grabs Marty, then the two of them beat up Marty and pour booze down his throat.
McGarrett and Danno visit the brutally beaten Mavis at the hospital. McGarrett wonders, "how could anyone do that to another human being?" The doctor says her condition is bad, that she has been unconscious since she was brought into the hospital. She will survive, but she'll need plastic surgery.
Marty wakes up in an alley surrounded by booze bottles. He picks one up and smashes it against the wall. He calls Gloria to come and get him, but then wanders into Soonie's Lounge. McGarrett arrives at the AA center, saying that Marty is in trouble.
Marty is finally located at his house, and McGarrett grills him, thinking that he is responsible for the beating of Mavis. McGarrett throws Marty in the shower and sobers him up with coffee. Marty tells McGarrett the story of the knockout drops after McGarrett threatens to book him. McGarrett says that Marty has lost his grip. Finally Marty says he'll tell McGarrett everything.
Salazar tells Sakai he has found the "weak spot in Fort Knox." A catering truck comes every couple of hours, and when the employees come outside, Sakai can sneak in and rig the truck similar to the last robbery.
Duke tells McGarrett there are witnesses to Kegel's accident, but they all have different stories. McGarrett wants to get Salazar for more than assault -- he wants him for murder and grand theft.
Wearing a welder's helmet, Sakai enters the armored car company and starts to work on the truck.
At the hospital, Marty apologizes to Mavis -- he says he'll be back to take her out.
Marty visits Five-O and says he wants to help McGarrett catch Salazar, that McGarrett should put the word on the street that Marty is gunning for Salazar, which will bring the crook out in the open. McGarrett pooh-poohs this idea. As Marty leaves, McGarrett says, "If you need anything, that door is always open."
Sakai's work to make carbon monoxide go from the exhaust manifold to the air conditioning system has been detected by the mechanics at Flynn's. McGarrett decides to send the truck out anyway, with Duke as the driver.
Later at Flynn's, as Duke is about to drive away, Marty appears despite McGarrett's objections. Marty distracts a guard, jumps in the truck and drives out of the garage, almost running over McGarrett in the process. He drives to the Coast Highway, followed by Sakai and Salazar who in turn are pursued by Five-O. Marty drives the truck erratically as if under the influence of carbon monoxide, and turns off the road. When Salazar approaches the door, Marty opens it and Salazar shoots him in the leg. Fortunately, McGarrett and Danno arrive to save the day.