At the agency, Lionel is pursuing his hobby of trying to solve old murders by "investigating" the Sable Hill Murder of the 1940s minor actress. E.L. is busy betting on horses and would rather not listen to Lionel's theories. When they go out, Lionel complains that murders are crude and common compared to the ones of decades past. As they walk down the street, a car tries to run E.L. down. When he and Lionel duck, the driver opens fire with a Tommy gun and then drives away. Lionel figures someone is out for revenge on E.L., but his partner insists he isn't running any current scams.
Sgt. Bogart of the LAPD investigates the assault and also believes it's someone trying to get revenge on E.L. Lionel comes to his partner's defense but Bogart suggests that E.L. ripped off one of his former partners and leaves. Disgusted, Lionel notes that Lieutenant Vernon Laws, the detective who handled the Sable Hills murder, wouldn't be so dismissive. E.L. would rather go to a bar and relax, and Lionel goes with him. As they drink, Lionel has to ask if E.L. is lying to him, and E.L. insists that he's telling the truth. They finally figure that it was just a random driveby shooting. However, as they stagger home, the car comes after them again in an alleyway and they just evade a hail of bullets.
The partners call Bogart to the office to file a report, but he still insists that E.L. is hiding something. When E.L. refuses to confirm anything further, Bogart starts to go but notices Lionel's board of clippings on the Sable Hill case. He echoes Lionel's earlier sentiments that they don't make crimes like that any more. Once the sergeant leaves, Lionel suggests that they go see Laws, who he has been working with and has long since retired from the force. E.L. prefers that they handle it on their own. They stand watches for the night and Lionel gets out his target pistol from the safe. Outside, someone plants a bomb in his car.
The next morning, the partners leave the office and panic when a car comes toward them. It's the wrong one, and nothing happens. They discuss the Sable Hill case and Lionel suggests that he and Laws turned up something, and someone wants to silence them. He admits that he called Laws despite E.L.'s wishes, and Laws says he turned up something on Jigs Duran, one of the key suspects in the murder. Lionel explains that he talked to Duran, and speculates that the killers were coming after him instead of E.L. E.L. offers to drive to Laws' house in the Mercedes Benz that he's scammed. First, Lionel checks his car and finds the bomb wired to the engine, but leaves it there for now.
At Laws' house, the partners find the door busted open. They break in and find Laws with a shotgun. He explains that someone shot at him through the door, but only grazed him. Laws figures that Lionel's theory that they've uncovered something is correct, and even E.L. is forced to agree. Lionel explains that Sable Hill died at Hansom Dam, and the gun believed to be the murder weapon was found later. However, the fingerprint card for the gun disappeared from the police evidence room, 40 years ago. Lionel found it during his investigation and gave it to Laws to check out. The ex-policeman admits that he's getting absentminded and seems to have misplaced it. As they prepare to go see Duran, Laws realizes that E.L. is an ex-con, because he knows the look. However, he is pleased to have E.L. along, admitting that crooks and policemen are playing the same game, just on different sides of the street. E.L. is pleased at his acceptance.
As they go to the hotel where Duran is staying, Lionel notes that Duran accused Laws of beating him 40 years ago during the investigation. Laws admits that Duran was correct, and that he hates modern-day police techniques and criminal rights. As they go up in the elevator, Laws gets out a piece of rubber hose, much to Lionel's surprise and shock. Lionel tries to convince him to put it away without success, since Laws figures he doesn't have anything left to lose. Duran is coming out of his apartment and opens fire, and escapes when Laws shoots back.
Bogart investigates the shooting, and knows Laws from the latter's day on the force. E.L. figures that Laws plans to sell them out, while Lionel is disappointed that his idol has feet of clay. Laws tells Bogart about Lionel's unlicensed target pistol and Bogart confiscates it. Lionel decides he can no longer trust Laws and plans to look for the missing fingerprint card himself. Disgusted, Bogart is tempted to arrest the partners but finally lets them go with a warning.
At the office, E.L. takes an interest in the Sable Hill case, but Lionel is no longer interested. E.L. prods him into reviewing the facts, and Lionel notes that almost everyone associated with the case died within two years of Sable Hill's murder, all of unnatural causes. He also notes that Duran is the one who told him about the missing fingerprint card, when Lionel interrogated him initially. Lionel goes back over the clippings and discovers there was a reference to a secret admirer. E.L. does a bit of deductive reasoning himself and concludes that Sable was a hooker. Lionel agrees that it's a possibility but they need to see the dead-case files to prove it.
E.L. comes up with a new scam and goes to a hobby shop to buy a model Air Force fighter. He uses the decals for his collection of fake ID badges and then they pose as Air Force MPs to get into the police station. They talk to Captain Matthews, the watch commander, who is suspicious of their claims. Lionel manages to distract her while E.L. makes a fake phone call to the Pentagon. Finally out of irritation, Matthews gives them access to the computer room. As they go, they notice someone who looks suspiciously like Stephen J. Cannell, author of the Mark Savage novels, but they're forced to move on before the police examine them too closely.
In the computer room, they get Gail, the technician, to bring up the files on Sable Hill. She flirts with Lionel, who almost forgets that he's posing as a USAF MP. They grab the info and quickly get out, but bump into Bogart. They ditch him temporarily and then pose as repairman to haul out a coffee vending machine. Once in the street, they abandon it in the middle of the road to block any pursuit and then escape in their car.
Once they're clear, the partners confirm that Sable Hill had a record as a prostitute. Laws was in Vice at the time, and busted her. Two months later, he was put in charge of the investigation into her death. They realize that Laws killed Sable, messed up the investigation, beat Duran into silence, and deliberately lost the fingerprint card. When Lionel reopened the case, Laws offered to help so he could stay close, stall his investigation, and eliminate Lionel if he got too close to the truth.
The partners return to the office to get Lionel's files. Duran calls and says that he needs to meet them at Hanson Dam so they can protect him from Laws, and he can tell them what he knows. Lionel agrees, unaware that Laws is forcing Dugan at gunpoint to lie. As the partners leave, the police show up. They bluff their way past two patrolmen, but Bogart pulls up. Lionel pulls the bomb out of his car and they escape.
At Hansom Dam, Lionel hides dynamite sticks on himself over E.L.'s objections. They go to the boathouse where Sable was killed 40 years ago. Duran comes down the stairs to the boathouse, and yells a warning. Laws opens fire and they take cover. Lionel tries throwing a stick of dynamite uphill with a marked lack of success. E.L. borrows a stick and throws it, but it rolls back down the hill before exploding. However, on the third throw they manage to stall Laws, knock him down the hill, and grab his gun.
Later, Lionel reopens the Black Dahlia case, much to E.L.'s dismay. He goes outside to the payphone, calls Lionel, and pretends to be Bogart to warn him off the case. Lionel believes him, but Bogart shows up to take their statement and arrests E.L. for impersonating an officer.