Ken Langston owns the Sands Motel in Gallitin, NM. He and his wife Diana check in a guest, Jerry Straiter. The next morning, Ken leaves for a fishing trip at 4 a.m. Someone attacks him a short time later and beats him to death. Police officer Sgt. Calhoun is driving by the docks at the lake when he spots Straiter preparing to dump Ken's body into the water. Straiter draws a gun and Calhoun is forced to shoot. As Straiter falls into the water, Calhoun sees a woman drive away in a car.
A cook is going in early for work at the Blue Moon bar and Grill across from the Sands Motel when he sees a woman pull up and go inside. Calhoun and other officers arrive at the motel and Diana comes right out even though it is early in the morning. They take her to the station and the officers, including sergeants Magruder and Mitchell, interrogate her about her husband's death. Diana explains that she was up because a drunken woman called and tried to find someone who wasn't staying at the hotel. When she says that she has no idea who Straiter is, the acting chief, Lt. Kelly Redfield, shows her records indicating that Straiter has stayed there twice in the last month. They've also learned that she drove to Dallas a week ago and Diana admits that she went to see a gynecologist because she and Ken were having trouble conceiving a child. Magruder suggests that she was having an affair with Straiter and went to Dallas to see him, and they lock her up. However, Calhoun warns Redfield that they can only hold her for 48 hours, and they eventually let her go. As Magruder drives her back to the motel, he tells her that everyone in town knows that she's guilty and that they'll eventually prove it.
Three months later, ex-cop turned PI Frank Cannon returns to his apartment in LA from a trip to Japan. His lawyer, Herb Mayer, comes in with his correspondence from the last three months he's been gone and they discuss his breaking up an extortion ring in Japan. As he goes through his correspondence, Cannon finds a letter from Diana about Ken's death. Cannon tells Herb that he and Ken worked in Counterintelligence together in Korea. He never met Diana but figures that she's innocent if she wrote to him and tells Herb that he's heading for Gallitin.
As Cannon drives into Gallitin, Magruder pulls him over for speeding. Magruder is willing to take a bribe and overlook the ticket, warning that Cannon will otherwise have to wait a week for the judge. Cannon tells the sergeant that he'll wait and asks for directions to the Sands Motel. Magruder tells the PI that he doesn't want to stay there but Cannon insists and heads out. As he checks in, Diana comes out to greet him and an obscene caller calls her. Cannon takes the phone and hangs up, and asks Diana if she's reported the caller to the police. When she admits that she hasn't, Cannon calls them and ends up talking to Mitchell. The sergeant isn't interested and tells Diana to ignore the calls.
Realizing who Cannon is, Diana thanks him for coming but admits that she wasn't expecting him. She says that she can't pay him but Cannon says that he came in part because Ken wrote to tell him that there some of the town government were crooked. Cannon goes back over what he heard when he took the call, and tells Diana that he heard bluegrass music in the background as well as a fan with a faulty bearing.
Cannon starts checking phone booths at all of the local bars and restaurants. He discovers that many of them are running gambling joints in the back. When Cannon pulls out after one stop, a truck driver, Jackie, cuts him off and the PI rear-ends him. A fight threatens to break out with the locals, who back Jackie, but Calhoun arrives and beaks it up. After Cannon's car is towed away, he goes back to the Sands Motel in a taxi, and talks about the town with the driver, Jake Hecker. Hecker knows about the murder and Straiter, and Cannon invites him to the Blue Moon for a drink. The owner, Eddie, welcomes them in, and Hecker talks about how Straiter went broke owning a bar and ended up selling alarm bells and security systems for a company in Dallas. According to rumor, Ken and Straiter had never met. Two toughs come over and get in Cannon's face, but he puts on a bluegrass song on the jukebox and then confirms the phone booth has a faulty fan. He then calls Dana and has her listen, confirming that the background noise was what she heard earlier.
That night, Cannon is at his motel room when a caller dials in and tells him to get out of town, and that's the only warning he'll get. The next day, Diana yells for help and Cannon comes over to the room she's checking out. The guest poured sulphuric acid all over the room and then snuck away. She explains that the guest checked in alone at 2 a.m. and gives Cannon the best description she can. Cannon calls the station and tells Mitchell to put Redfield on the line. While he waits, Diana finds the registration card and shows it to him, and Cannon realizes that the guest used Cannon's license plates. Redfield finally answers and Cannon tells him what happened. Once he's done, Cannon has Diana go over the other registration cards. The police have Straiter's cards but Diana remembers that Straiter put his company name, Electronic Enterprises, on the first two cards. However, the police never confirmed that he had a reason to be in town. The anonymous caller rings back and tells Cannon that the next time he won't give Cannon a warning.
Later, Cannon is working in the hotel room neutralizing the acid with bicarbonate soda when Magruder Arrives. He isn't impressed and tells Cannon that the acid will have destroyed any evidence and the vandal would have worn gloves. When Cannon asks to speak to his chief, Magruder says that his superior is being treated at the Mayo Clinic and Redfield is in charge. He also tells Cannon that his license plates are still on PI's wrecked car at the shop.
Cannon goes to the shop and confirms that the plates are there. Redfield comes in and points out that someone removed the plates and put them back on, oiling the screws with machine oil. When the lieutenant wonders why someone would have taken the plates, Cannon explains that the vandal wanted to send him a message because he knows the PI is on his trail. He accuses Redfield of not doing his job, but the lieutenant reveals that two weeks ago someone hijacked a shipment of sulphuric acid. Redfield insists that he knows there is a crooked element in town but that he's waiting to make his move. Cannon doesn't believe him and an angry Redfield takes his gun and his detective ID. When he points out that Cannon isn't licensed to practice in New Mexico, Cannon says that he isn't taking any pay. Redfield orders him out of town but Cannon says that he isn't going anywhere.
Cannon goes back to the Blue Moon and asks Eddie if he thinks Diana killed Ken. Eddie doesn't want to commit himself but explains that Ken was a hometown boy that everyone liked. He suggests that Ken ran afoul of criminals but not the crooked element in town, and admits that the case against Diana looks strong. Cannon explains about the obscene phone call from the phone booth, and Eddie tells him who was there at the time. One of the men was a local character, Virgil Holley, but Eddie warns him that he's not the hillbilly-type that he resembles.
Cannon goes back to his motel room and gets a call from an anonymous woman who wants $100 to tell him where to find the acid. She starts to make arrangements for pay but then hangs up abruptly. Cannon runs over to the bar but there's no sign of any woman in the bar, and Eddie confirms that no one has come in since Cannon left a few minutes ago. Virgil comes in and comments on Cannon's near-fight with Jackie on the street. The man tries to get Eddie and another man interested in a hermit crab race but they figure it's a sucker bet and walk away.
That night, Cannon is in his room when the woman from earlier calls. She explains that she almost got caught and asks for $500 to give Cannon the name of the vandal and the name of the man who hired him. Cannon asks for proof and she gives him the address where he can find the stolen acid. Cannon drives out to the abandoned farm the next day and starts to go up a ladder to the attic, but someone fires at him with a shotgun, hitting him in the left arm. Cannon manages to bust through the back door and escape in the borrowed motel car.
Cannon goes to the hospital and gets the gooseshot removed, and the doctor reports the shooting. Magruder and Mitchell bring Cannon to the station and interrogate him until Redfield tells them to check out his story. Redfield has confirmed that Cannon is a former detective-lieutenant and Cannon admits that he didn't work well under authority. The PI figures that the people responsible are trying to get him out of the way and drive Diana out of town. Redfield is clearly upset about something and Cannon asks what's eating at him and walks away.
That night, Diana treats Cannon's wound and explains that she was a former nurse, which is how she met Ken. He just wanted to move back to his hometown and settle in, but was upset because he discovered how corrupt Gallitin had become. Cannon figures the same people are trying to run Diana out of town so she can't reveal what she knows, but she points out that she doesn't know anything.
The next day, Cannon goes to see Redfield at his home and finds his wife Christie sunbathing on the back lawn. She smiles seductively at Cannon, who hears Redfield approaching and goes to greet him. The lieutenant initially doesn't want to discuss business at home, while Cannon studies his tires. They go in the back and discover that Christie has gone inside, and Cannon explains that he drove back to the farm and found tire treads with a diamond pattern that don't match Redfield's. He did find a cigar and points out that it matches the one that Redfield is currently smoking, but the lieutenant says that he wouldn't have been dumb enough to leave evidence. Christie comes out, dressed, and says that she's heading into town. She offers the two men drinks and welcomes Cannon, commenting that he looks familiar.
Cannon goes shopping with Diana and they discuss the case. She explains that Ken and Redfield were fishing buddies and friends since high school when Redfield idolized her husband. They were supposed to go fishing the morning of Ken's death but Redfield called her to say that he was cancelling because he had to extradite a prisoner. Ken had already left by then so he left a message. She explains that Redfield and Christie married two years ago before she and Ken did, and that Christie was originally a teacher from Warren Springs. Virgil is Christie's cousin and Diana explains that Virgil might look like a hillbilly but he owns most of the valuable real estate in Gallitin. She's sure that the eloquent speaker on the phone definitely wasn't Virgil. As they get back to the motel, Cannon wonders if Christie knew that her husband cancelled the fishing trip, and points out that someone could have run from Redfield's house to the motel in two minutes. The PI figures that Redfield walked in on something and wonders if he was having an affair with Christie. Diana points out that Redfield is a jealous man and would have killed Christie, not Ken, if he had found about an affair between them.
Cannon and Dian drive to Warren springs and visit the school where Christie taught. They talk to a teacher who explains that Christie's first husband died three years ago in a bathroom "accident." She then gave up teaching and spent some time in Dallas. Cannon figures that Christie and Straiter met back up in Dallas. As he talks to Diana, the school bell goes off and Cannon realizes that it belongs to Electronic Enterprises collected the insurance money, then joined back up with Straiter afterward. Straiter followed Christie to Gallitin to continue their partnership, and Ken walked in on them that morning so they had to kill him. As they drive, a pickup truck sideswipes them. Cannon drives after it but the driver drops bales of hay off the back and drives them off the road. Diana and Cannon realize that someone must know that they went to Warren Springs to trace Christie.
As Cannon drives back through Gallitin, Diana spots the vandal and Cannon sends her to the motel while he goes after the man. After checking a few bars, Cannon finds the one where the man is hiding out. Magruder is there and watches as the bar brawlers come after Cannon, who fights back. When the brawlers go for knives and broken bottles, Calhoun arrives and knocks Cannon out from behind.
When cannon wakes up the next morning, he finds himself at Calhoun's house. Calhoun apologizes, saying that he tried to be gentle but that they would have killed him if he hadn't interfered. He's interested in knowing why Cannon went to Warren Springs but the PI refuses to answer. Calhoun defends Redfield, saying that he's an honest man, and has come to the same conclusion about Ken, that he saw Straiter and Christie together the night he was murdered. He's made sure that that Redfield was out of town at the time of Ken's death just as he claimed.
The two men walk around the lake to the spot where Calhoun shot Straiter. Calhoun and Cannon both figure that Straiter was trying to make it look a fishing accident rather than dump the corpse in a ditch, and wonder why anyone thought Straiter had to make sure the trail led back to them. Cannon concludes that Ken showed up at the Redfield house in fishing clothes so Christie figures that he hadn't got the message she gave Diana. Since Diana knew about the message, she could incriminate Christie, so Christie has been trying to drive Diana out ever since. Cannon wonders why Straiter had a gun when he was disposing of a corpse and hadn't expected Ken at the house. He tells Calhoun to find out if there were any other crimes committed that night, and the sergeant confirms that a gang in the adjoining state robbed two store safes. The PI asks Calhoun to find out which alarms the two stores used.
When Calhoun drops Cannon off at the motel, he discovers that Diana is gone. He calls Redfield at home but gets Christie, who says that her husband is out. She tells Cannon that a man came by to talk to her husband about Diana and Cannon asks her to hold him there. When he drives there, Christie says that the man left to find Redfield and offers Cannon a drink, closing the drapes. She then calls the station, asks for her husband, and starts screaming for help. Christie then hangs up and rips her dress, telling Cannon that her husband will be there in two minutes. When Cannon refuses to run, Christie throws himself at him. The PI shoves her away and tackles Redfield when he comes through the door, gun drawn. The two man struggle and Cannon finally knocks out Redfield and takes his gun. Disgusted, he tells Christie she can drop the loving-wife act and drives away in Redfield's police car.
Cannon goes to the Blue Moon and Eddie tells him that Diana has been calling all over town to find Cannon. Someone called her to the hospital, saying that Cannon had been injured in an ambush. Virgil is at a nearby table entertaining his friends with some fake accents, and the PI gets an idea and leaves.
Diana pulls up and Cannon says that they have to get out of there before Redfield issues an APB. They drive to Virgil's house and Diana insists on going in with Cannon. They approach the house and Cannon has Diana wait outside while he goes in. Jackie and the vandal are inside with two women, and Cannon gets the drop on the two men. He ties them up and dumps them in the trunk of their car while Diana watches the women. Cannon confirms that they're Laverne Holley and Trudy Holley as he ties them up. When Laverne starts talking about her uncle and Trudy tells her to shut up, Cannon recognizes Trudy's voice as the woman who called him on the phone. He notices a nearby fan and turns it on, confirming that it has the same faulty baring as the one at the Blue Moon.
Cannon calls Calhoun and confirms that Electronic Enterprises installed the alarms at both robbery sites. The PI tells Calhoun that Straiter set up the robberies with his inside knowledge, which is why he had the gun that night. Cannon then tells the sergeant to inform Redfield that he's turning himself into the FBI to tell them about the interstate robberies, and will turn himself over to Redfield afterward. The PI then forces Trudy at gunpoint to make a new call.
Virgil is at the Blue Moon when Eddie tells him that he has a call. Trudy tells him that Jackie found out that Cannon knows about the robberies so they're leaving town. Virgil and his accomplice, Magruder, drive to the house and Virgil goes inside to stop them from running while Magruder waits outside. Cannon gets the drop on Magruder while Virgil takes out the stolen money and diamonds from his safe. The PI comes in behind him and captures Virgil, who admits that he controls the entire town. He offers to split the robbery take with Cannon, who figures that they were breaking open the safe that morning at Redfield's house when Ken came in.
Christie arrives and gets the drop on Cannon, telling him to drop his gun. However, she also holds Virgil captive and blames him for falling for Cannon's trap when the PI didn't have any evidence. Virgil claims that he didn't get much money from the take but Christie realizes that her cousin is double-crossing her. She blames Virgil, saying that she and Ken only wanted to collect their share and get out. She thinks that Virgil killed Straiter but Cannon explains that Calhoun fired the fatal shot.
Diana comes in and, realizing what happened, turns off the lights. Virgil takes advantage of the distraction to jump Christie, and Cannon knocks him out and grabs Christie's gun. Redfield comes in and Christie tries to appeal to him as her husband, but he forces her to hand over the diamonds and then arrests her.
Sometime later, Cannon and Calhoun walk out of the courtroom. Virgil has been convicted and had enough evidence in his safe to put away the city manager and the chief of police. The robbers asked Virgil to help them launder their money and fence the diamonds, but he got greedy and kept it all. As Cannon goes to pay his traffic ticket, Redfield comes over and apologizes for wanting to kill him, but Cannon assures the lieutenant that he had eighty pounds on him and it wasn't going to happen. Diana is waiting for them and Redfield offers to drive her home, and she gives Cannon a thank-you kiss on the cheek and leaves.