Irwin M. "Fletch" Fletcher is a newspaper reporter being offered a large sum to off a cancerous millionaire, but is on the run, risking his job and finding clues when it's clear the man is healthy.
Irwin "Fletch" Fletcher, Los Angeles journalist, really lives for his profession. As Jane Doe, he publishes articles that have caused several heads to roll in the past. Now, Fletch is at it again: In disguise as a bum, he lives at the beach, researching drugs and their dealing. One day, Fletch is addressed by Alan Stanwyk, a rich man, who asks him, the bum, a favour. For the sum of $50,000, Fletch should kill poor cancer-ridden Mr. Stanwyk with a gun, so that his wife will get the insurance money. What the guy didn't think of was Fletch's real profession. Returning into normal life, Fletch instantly takes up research not only to find out that Mr. Stanwyk is healthy as life itself but he also runs into certain connections between drug dealing at the beach, Alan Stanwyk, his private jet, the police and a very expensive piece of Land in Utah.
Fletch is an ace reporter. While working on a story on Drug sales, he is approached by Alan Stanwyck who wants Fletch to murder him to save him from a lingering cancer death. Fletch begins to nose around and finds irregularities in Stanwyck's past and Underhill's Visa Number.
- Los Angeles Times reporter Irwin "Fletch" Fletcher (Chevy Chase) is writing an article exposing the drug traffic on the beaches of Los Angeles. Posing as a homeless addict during his investigation, he is approached by Alan Stanwyk (Tim Matheson) who mistakenly assumes Fletch is a junkie. Taking him to his luxury mansion in Beverly Hills, Stanwyk formally introduces himself and that he works for Boyd Aviation as the executive vice president. Stanwyk claims to have bone cancer, with only months left to live, and wishes to avoid the pain and suffering. Stanwyk offers $50,000 for Fletch to come to his mansion in a few days time, kill him, and then escape to Rio de Janeiro, staging the murder to appear as the result of a burglary.
Fletch is suspicious but ostensibly agrees to the plan. Along with his colleague Louisa Larry (Geena Davis), he begins investigating Stanwyk instead of completing his drug trafficking exposé, much to the dismay of his impatient boss and editor Frank Walker (Richard Libertini).
Fletch endures a rectal exam from Stanwyk's doctor, Dr. Dolan (M. Emmett Walsh), in a failed attempt to talk Dolan into divulging medical information about Stanwyk. Later that same day, disguised as a doctor, Fletch accesses Stanwyk's file at the hospital and learns Stanwyk lied to him about having cancer.
Fletch visits Stanwyk's wife Gail (Dana Wheeler-Nicholson) at her tennis club and, pretending to be a friend of her husband's, flirts with her during an impromptu tennis lesson. He doesn't uncover very much from her and leaves.
Next, Fletch poses as an FCC agent where he visits Gail's father, Stanton Boyd, who is the head of Boyd Aviation to inquire about Stanwyk's finances. Fletch finds that Gail recently converted $3 million of her personal stock in Boyd Aviation into cash for her husband to buy a ranch in Provo, Utah.
Fletch returns to his apartment for the day and finds that his ex-wife's lawyer Marvin Gillet (George Wyner) is waiting for him to collect unpaid alimony. Fletch gives Gillet the $1,000 cash bribe money that Stanwyk gave him earlier to make him leave. Fletch then goes to sleep and has a dream of being a major league basketball player playing for the Lakers and scoring a hoop.
Fletch travels to Utah where he breaks into the realtor's office and discovers the deed lists the sale price as only $3,000. He barely escapes from the office while being chased by a guard dog.
Meanwhile, LAPD Chief Jerry Karlin (Joe Don Baker) gets wind of Fletch's soon-to-be-published drug article. He sends two thuggish detectives to Fletch's apartment and they arrest him after they plant heroin on him. The two take Fletch to meet with Karlin, who tells Fletch he's jeopardizing the lives of undercover cops he's planted on the beach to bust the drug trade. When Fletch insults Karlin by smashing a picture of Karlin with Tommy Lasorda, Karlin threatens to shoot Fletch and place a switchblade in Fletch's dead hand to claim self-defense. Fletch tells Karlin he's going to work on another story about off-track betting in the Himalayas instead. Karlin is amused and lets Fletch go. Fletch finds out that Karlin had Fletch arrested at the request of Fletch's boss, who wants Fletch to stop investigating what he considers a ludicrous story about drug-dealing cops. When Fletch confronts his boss about the incident, Frank refuses to believe Fletch's story about Karlin's plan to kill him. Fletch storms off, angry but more determined than ever to break the story.
Disguised as a redneck airplane mechanic, Fletch shows up at Stanwyk's plane hangar and learns from two mechanics that Stanwyk has been using the plane to travel back and forth between California and Brazil on a weekly basis for several months.
Back at the tennis club, Fletch meets Gail at her cabana. He treats her to an expensive lunch which he charges to the credit account of a snooty and obnoxious club member, Underhill, whom Fletch'd overheard being abusive toward the staff. Fletch gives her a little information about the investigation but has to flee when Mr. Underhill shows up at Gail's door, demanding the several hundred dollars the lunch had cost.
Fletch spies on Stanwyk making a suspicious briefcase exchange with Chief Karlin, but is unable to determine the nature of their relationship. He returns home to find LAPD officers lying in wait at his apartment and flees and manages to avoid being caught after a harrowing car chase when Fletch steals a Alfa Romeo from a young car thief and roars off, with several police cruisers and cycle officers in pursuit. He manages to stay ahead of them and is only able to shake them when he distracts them all at a lodge club dinner.
Fletch returns to Provo to hide out and, under the guise of an insurance investigator, interviews Stanwyk's parents. Fletch discovers Stanwyk has been secretly married to another woman for eight years, having used Gail for her money. He returns to Beverly Hills and shares what he's found with Gail, who's very upset about her husband's bigamy. Fletch asks her not to call the police until he can confront her husband when he goes to their home to carry out the murder he'd originally been hired for.
Fletch arrives at Stanwyk's mansion on the night of the plotted murder only to find Stanwyk armed and ready to kill him instead. In the trunk of Stanwyk's car Fletch finds several cans of gasoline. Fletch reveals he is aware Stanwyk's real plan was to fake his own death by killing Fletch and burning his body beyond recognition and dressing like Fletch himself, all detailed in a statement that Fletch's paper will print if Fletch doesn't report for work the next day. Stanwyk would then escape to Brazil with his other wife as well as Gail's $3 million. Stanwyk was also using his private jet to smuggle cocaine from South America to supply Chief Karlin who in turn profited by blackmailing ex-convicts like Fat Sam (George Wendt) to peddle drugs on the beaches. Chief Karlin unexpectedly shows up and, upon learning of Stanwyk's intentions to flee with $800,000 of the Chief's drug money, kills Stanwyk. Chief Karlin then turns the gun on Fletch but is knocked unconscious by Gail when she clubs him with a tennis racket.
Some time later, Chief Karlin is indicted due to Fletch's investigation. Fletch begins dating Gail and in the final scene, takes her on a vacation to Rio, Brazil. Fletch jokes that he'd placed the expenses for the vacation on Underhill's credit account and offers the audience the number.