Ace Ventura is an eccentric, very unorthodox Miami-based private detective who specializes in retrieving tame or captive animals. He struggles to pay his rent, and is often mocked by the Miami Police Department, led by Lieutenant Lois Einhorn, who finds Ventura insufferable. Two weeks before the Miami Dolphins are to play in the Super Bowl, their mascot, a bottlenose dolphin named Snowflake, is kidnapped. Melissa Robinson, the Dolphins’ chief publicist, hires Ventura to find Snowflake.
Searching Snowflake's tank for clues, Ventura finds a rare triangle-cut orange amber stone, which he recognizes as a part of a 1984 AFC Championship ring. Ace suspects billionaire Ronald Camp may have stolen Snowflake, as he is known for collecting exotic animals through less-than-reputable means and sources. Ventura and Melissa sneak into Camp's party, where Ventura mistakes a shark for Snowflake and is nearly eaten. Camp apologizes and shakes Ventura's hand, revealing on one of his own fingers an amber stone identical to the one Ventura found. Ruling out Camp, Ventura concludes that a member of the 1984 Miami Dolphins line-up may have kidnapped Snowflake, and attempts to identify the culprit by their rings. However, he discovers all of the team members’ rings are intact.
Roger Podacter, the team's head of operations, mysteriously dies after falling from his apartment balcony. Einhorn declares it a suicide, but Ventura proves that it was murder. He comes across an old photograph of the football team, discovering an unfamiliar player named Ray Finkle, who was only added in during midseason. Finkle missed the field goal kick at the end of Super Bowl XIX, which cost the Dolphins the championship, ruining his career.
Visiting Finkle's parents, Ventura learns that Finkle fully blames Dan Marino for the end of his career due to Marino allegedly placing the ball incorrectly before the kick, and was subsequently committed to a mental hospital for homicidal tendencies. Marino is kidnapped himself shortly thereafter. Ventura visits Einhorn, pitching his theory that Finkle kidnapped both Marino and Snowflake in an act of revenge, since the dolphin has been given Finkle's old team number and a goal trick to boot. He also theorises that Finkle murdered Podacter. Einhorn compliments Ventura and kisses him.
Ventura and Melissa go to the mental hospital, the former posing as a potential patient, where he uncovers a newspaper article in Finkle's possessions about a missing hiker named Lois Einhorn. Ventura, with a clue from his dog, realizes that Einhorn is in fact Finkle: Finkle used the fact that the actual Einhorn was missing and presumed dead (with no body found), and took on her identity, had surgery to change his gender, and began a career with the Miami Police Department to eventually get revenge on Marino and the Dolphins. On Super Bowl Sunday, Ventura follows Einhorn to an abandoned yacht storage facility where she has Marino and Snowflake held hostage. Einhorn calls the police, framing Ventura for the kidnappings. Melissa and Ventura's friend, police officer Emilio, stage a hostage situation to get the police to listen to Ventura.
Ventura strips Einhorn of her clothes to expose her failure to completely change her sex, but fails until Marino points out a bulge in the back of his underwear, actually Finkle's unchanged privates hidden out of view. This confirms that Finkle murdered Podacter after the latter had discovered Finkle's secret. Einhorn is arrested by the police after attacking Ventura, and Finkle's ring is identified to have a missing stone. Marino and Snowflake are welcomed back during half-time at the Super Bowl in a match between the Dolphins and the Philadelphia Eagles. Ventura tries to retrieve a valuable albino pigeon, but it is scared off by the Eagles’ mascot Swoop, causing Ventura to attack him in retaliation.