Tom Ripley is a secretive, troubled young man in New York. He makes his living by scamming elderly people and artists, but lies to his group of friends, fabricating careers and talents in order to seem accomplished. He has many acquaintances but no close friends, and his parents have been dead since he was young. Tom resents the aunt who raised him in Boston, though he relies on the small checks she sends him. One day, Tom is cornered in a bar by Herbert Greenleaf, the wealthy father of his friend Dickie. Herbert is under the impression that Tom and Dickie were once very close, although Tom barely remembers Dickie. Mr. Greenleaf is distraught because his son has gone to live in an Italian village and refuses to return home, even though his mother—Herbert Greenleaf's wife—is very ill with leukemia. Greenleaf asks Tom to write to Dickie and ask him to return, suspecting that Tom will have more influence than his parents. As their conversation continues, Greenleaf's request transforms: he wants to pay for Tom to sail to Italy and speak with Dickie in person. Tom agrees, tempted by the offer of money and travel. After a dinner at the Greenleafs' apartment, in which it becomes clear how desperate Dickie's parents are for their son to come home, Tom sets sail.
His arrival in Mongibello, Dickie's town, is awkward. Dickie does not remember Tom, and he has a close friend already, an American woman living in Mongibello named Marge Sherwood. Though Dickie evidently does not return Marge's romantic feelings for him, their friendship seems not to have space for Tom. Slowly, Tom charms Dickie, gaining his trust by telling him the truth about his reason for coming to Italy. Dickie even asks him to move into his house instead of staying in a hotel, and the two men travel through Italy together. Tom and Marge distrust each other and compete for Dickie's affection, especially since Tom seems to have unrequited feelings for Dickie too. This conflict slowly boils over, and Dickie, who is well-meaning but somewhat uncomfortable with emotional entanglement, ends up fighting with both Marge and Tom. One day, Dickie walks in on Tom trying on his clothes and mimicking his voice. Dickie loses his temper, accusing Tom of being homosexual, and explaining that Marge believes he is. Tom denies this, but is wounded. Soon after, he takes a trip to San Remo with Dickie, though Dickie is now reluctant to spend time with Tom. On the trip, Tom realizes that he could kill Dickie, assume his identity, and steal his money and possessions. Without thinking about it much, Tom kills Dickie while the two are sailing in a rented boat. He takes Dickie's rings off of his fingers, weighs down both the boat and the corpse, and abandons them in the ocean. Then he returns to Mongibello, telling Marge that Dickie has decided to spend some time in Rome and has asked Tom to send along his things. Tom packs some of Dickie's possessions and moves to Rome, where he begins to dress as and act like Dickie. He finds this exhilarating, since he now feels like his charming, wealthy friend instead of his own mundane and secretive self. He writes Marge letters, pretending to be Dickie, in which he explains that he needs space and does not wish to see her for some time. He also forges various documents in order to obtain Dickie's money for his own uses.
One of Dickie's old friends, Freddie Miles, is also living in Italy. Freddie, who has met Tom only briefly before Dickie's death, decides to visit Dickie in Rome. He is surprised to find Tom alone, wearing Dickie's clothes and rings in what he believes in Dickie's apartment. Tom explains that he is visiting Dickie, who has gone out. Freddie leaves, and Tom hears him conversing with the landlady downstairs. The landlady tells Freddie that "Mr. Greenleaf" is alone in his apartment and that he has not left all day. Freddie returns to confront Tom, who impulsively murders him. Tom them elaborately stages a scene of revelry in the apartment, leaving half-drunken cups of alcohol and cigarette butts scattered about. He carries Freddie's body outside to Freddie's car, trying to look as if he is merely supporting a drunk man. He drives the body to a location outside of town and abandons it. The police find the body soon after, and question Tom about it, believing that he is Dickie. Tom, as Dickie, says that he was drinking with Freddie on the day of his death, but Freddie left his apartment in the evening sober enough to drive. The police do not seem suspicious, but soon after, Tom reads in the newspaper that a bloodstained boat has been discovered in San Remo. He realizes that the police might find Dickie's body as well. If so, they will assume that the body is Tom's and believe that Dickie has killed both men. Furthermore, Dickie's bank has discovered what they believe are several forged documents, and Tom is concerned that this will cause heightened suspicion and could be traced back to him. Wanting to rid himself of Dickie's identity before this happens, Tom flees to Palermo and returns as "himself," with his own clothes, name, and personality. But, unable to bear parting with Dickie's luxurious possessions, Tom stashes Dickie's rings in a box and ships all of Dickie's other possessions to the American Express in Venice, where he leaves them for the time being. When he returns from his trip to Palermo, he learns that the police are seeking Dickie Greenleaf, who they believe has gone missing. In fact, the Italian press has begun sensationally covering the story, making Tom into a minor celebrity by association. He moves into a grand house in Venice and cultivates a new circle of friends, most of them international travelers. While he dislikes acting like himself instead of Dickie, he enjoys his new status and the attention he garners as the friend of a missing person.
As the search for Dickie heats up, Marge comes to visit Tom. Tom dislikes her more than ever, but tries to preserve the appearance of affection for her while they attend parties and discuss Dickie's disappearance. Tom tells Marge that he believes Dickie committed suicide. Marge disagrees at first, but accidentally finds Dickie's rings in Tom's apartment. Tom tells her that Dickie gave him the rings in Rome before his disappearance, explaining that he wanted Tom to keep them in the event anything happened to him. This explanation convinces Marge that Dickie committed suicide. Shortly after, Mr. Greenleaf flies to Italy. He seems resigned and angry, unsure whether his son is dead or merely hiding away. He brings an American private investigator named Alvin McCarron, but, though McCarron questions both Tom and Marge, he can find no helpful evidence and returns to America with Mr. Greenleaf. Tom, meanwhile, forges Dickie's will, which claims that Dickie wants all of his money and possessions left to Tom. He places the will in an envelope, on which he writes instructions that the envelope should not be opened until June. When June comes around, shortly before leaving on a trip to Greece, Tom pretends to discover the will and writes Mr. Greenleaf a letter describing its contents. This is a calculated risk, but, just after sending the letter, Tom learns that the police have discovered Dickie's possessions in storage at the American Express. He worries that his fingerprints will be found on the luggage, and that this, in conjunction with the forged will, could be his downfall. He leaves for Greece convinced that he will be arrested the moment he docks. Yet when he arrives in Greece, he is not arrested. He finds a newspaper, and an article about Dickie's case reveals that the fingerprints on the luggage match those in Dickie's abandoned Roman apartment. This has lead investigators to conclude, incorrectly, that both sets of prints belong to Dickie, and that he likely committed suicide. Tom goes to the local American Express to pick up his mail, and finds a letter from Mr. Greenleaf, in which Greenleaf says that he is unsurprised to hear about his son's will, and says that all of Dickie's wealth should be left to Tom. Unable to believe his good fortune, Tom hails a cab and asks the driver to take him to the town's best hotel.