The movie begins with our hero Ace Ventura (Jim Carrey) rescuing a dog from an angry man, all but destroying his car in the process. The dog's owner, an attractive young woman, seduces Ace to avoid paying the detective's fee- a feat that Ace hardly resists.
Late in the night at Miami Stadium, a group of individuals break in and steal the Miami Dolphins' mascot- a trained dolphin named Snowflake. Next morning, the Dolphin's Head of Operations Roger Podacter (Troy Evans) and his Chief Publicist named Melissa Robinson (Courtney Cox) are instructed to find the missing Snowflake before the upcoming Super Bowl (out of fear that the theft will affect the superstitious football players). A member of their staff was previously a client of Ace Ventura so they decide to call in this "pet detective" in to try investigating.
Meanwhile, Ace is trying to capture a rare bird with a reward of $25,000 with no success. After receiving the request from Melissa to investigate, Ace goes to the stadium and takes a look inside Snowflake's tank, finding a small stone in the filter system. Roger spots a team of reporters heading to the tank and is worried (fearing the negative press that would result from knowledge of Snowflake's abduction). Ace poses as an eccentric animal trainer and manages to drive the reporters (Antoni Corone and Margo Peace) away.
Later that day, Ace Ventura stops in at the local police headquarters. After an altercation with Sgt. Aguado (John Capodice), Ace meets his friend Detective Emilio (Tone Loc) to find out what leads the police may have. Emilio admits that a missing animal case isn't a priority for the police but they suspect that one of the local animal rights groups, who have repeatedly protested the Miami Dolphin's ownership of Snowflake, may be responsible. At this point, Emilio's superior, attractive female Lt. Lois Einhorn (Sean Young) comes along and throws Ace out.
Ace visits his hacker buddy Woodstock (Raynor Scheine) to try and find information relating to the purchase of equipment that would be necessary for handling a dolphin. Woodstock notices several purchases by a man named Ron Camp (Udo Kier), a billionaire who apparently enjoys gathering rare fish. Furthermore, Woodstock notes that Camp donated land to the Miami Dolphins for the building of a new stadium.
Ace decides to investigate Ron Camp further and gets into a party thrown by Camp that evening with Melissa. As Camp attempts to seduce Melissa, Ventura investigates Camp's home and finds a large aquarium tank that he assumes holds the missing Snowflake. However, the tank actually holds a large shark and Ace barely escapes with his life.
As Melissa and Ace leave the party, her very annoyed and him wet as a hen and with rags instead of pants, the detective notices an unusual ring on Camp's hand, manhandling the billionaire in order to get a closer look. When they return to the stadium, Ace informs Melissa of the stone he found in Snowflake's tank-and Camp's ring had the same kind of stones, but since no stones were missing, Camp was not the one responsible for Snowflake's abduction. The ring in question is a 1984 AFC Championship Ring, won by the Miami Dolphins. Ace decides to personally investigate all the ring-owners to find one with a missing stone, but before long his photographic list of suspects has run out with no results.
Melissa attempts to console Ace about his failure to find the damaged ring. Ace is in no mood for her and they trade insults. As Ace goes inside to apologize, Melissa receives a devastating phone call- Roger Podacter is dead.
The police investigate Podacter's apartment, learning that Roger's body was found after crashing through a sunroof twenty stories below his apartment balcony. Lt. Einhorn is in charge of the investigation and she is prepared to pronounce Podacter's death a suicide. Ace, however, disagrees, insisting that Roger was murdered. After interviewing Podacter's neighbor (Florence Mistrot), who had heard a scream in the apartment and the manager (who opened the balcony door to check the place), Ace's theory is proven- Roger's balcony door is made of soundproof glass and thus Roger's neighbor couldn't have heard a scream with the door closed- meaning that somebody else closed the door upon leaving the apartment after throwing Roger Podacter over the balcony.
Despite proving his theory to the police, Ace is still upset about not finding the correct ring. Melissa takes Ace to the stadium office to investigate paperwork regarding the sale of the rings. There, Ace spots an unfamiliar player face that was not in his previous checklist. Melissa tells Ace that the player is a mid-season replacement kicker named Ray Finkle. Furthermore, Finkle is the player who allegedly kept the Dolphins from winning the 1984 Super Bowl- by missing the final field goal in an event known as "The Kick Heard Round The World", a reference to Scott Norwood's missed field goal kick in the closing moments of Super Bowl XXV. The result was Finkle's contract with the Dolphins being canceled and he never played football again.
Ace drives to Finkle's childhood home and finds it covered in graffiti and damage from the still-bitter townsfolk. Finkle's father (Bill Zuckert) is outwardly hostile, pointing a gun at Ace until Mrs. Finkle (Alice Drummond), senile yet doting over her absent son. Ventura learns that Ray Finkle was institutionalized in a private hospital several years ago, but escaped soon after. A tour of Finkle's room shows a testament to his hatred of Dan Marino (Dan Marino himself), the player who held the ball for Finkle's Kick and who Ray blames for the whole thing insisting that Marino held the ball "laces in" instead of out, per regulations.
Realizing that Marino is likely in danger, Ace telephones Melissa and requests extra security for the star player. Marino is filming a commercial for Isotoner gloves when he is tackled by two men in Miami Dolphins uniforms-apparently as part of the commercial. But when the "Dolphins" keep carrying Dan after the director yells CUT, everyone realizes what has happened. Ace attempts to stop the abductors but they evade capture.
Einhorn finds Ace in her office after a press interview, infuriating her- until Ace mentions the name Ray Finkle. Ace shares his theory with the lieutenant- Finkle has come back to take his revenge right at Super Bowl time, when it will be most harmful to the Dolphins. Snowflake was the first target of revenge because the dolphin had been given Finkle's old number and taught to kick a field goal as part of the halftime show. Ace also theorizes that Podacter was murdered because he recognized Finkle from his previous tenure with the Dolphins. Lt. Einhorn actually applauds Ace's work, making physical advancements towards the pet detective- which Ace rebuffs.
Desperate to find out more about Ray Finkle, Ace goes to the mental hospital where the ex-player was held in an attempt to search through any remaining belongings. Melissa poses as an anguished sister looking to commit her deranged, football-obsessed brother to give Ace a chance to explore the hospital. She talks to the doctor (David Margulies) while Ave looks into every room in the hospital. A check of the storage room reveals further examples of Finkle's hatred of Dan Marino along with an unusual news article- the failure of search parties to locate a missing hiker named Lois Einhorn.
Emilio checks out Lt. Einhorn's desk at police headquarters, finding a love letter to the lieutenant from Roger Podacter dated several days before Rogers' death. Ace struggles to find the possible link between Finkle and Einhorn but cannot see any viable connection. When Ace's pet dog lies on a picture of Finkle (covering the player's short black hair with long red threads) Ventura finally realizes the truth- Finkle IS Einhorn! Ace is overjoyed at finding the answer, but repulsed by the knowledge that the lieutenant who attempted to seduce him is actually a man.
On the morning of the Super Bowl, Ace tails Einhorn to a remote boathouse where Dan Marino and Snowflake are being kept. Ace manages to defeat Einhorn's two henchmen but is caught by Einhorn and held at gunpoint before he can free Marino. Einhorn radios the police for assistance, claiming Ace is the one behind the abductions.
Einhorn is distracted by a news report on the Super Bowl, repeating the '84 "Kick Heard Round The World" clip. In her rage, Ace is able to overpower her and Snowflake takes the lieutenant's gun away. As the police arrive, Einhorn orders them to shoot Ace- until Melissa arrives holding Emilio at gunpoint to distract the other police officers enough to let Ace finish the case.
Ace explains the full details of Finkle's new career and plan, ripping off Lt. Einhorn's clothes in an effort to prove his point. However, Ace's efforts only seem to prove that Einhorn is a genuine female. However, Dan Marino spots the truth- Einhorn has male genitalia tucked between her (his?) legs. Ace realizes that this is why Roger Podacter died- he learned of Einhorn's true gender!
Enraged, Einhorn attempts to stab the pet detective but Ace knocks her into the water with Snowflake. Einhorn staggers back out (her red hair now washed back to black thanks to the water) and Ace notices a ring on her finger. Ventura leans over and cheers in triumph- it is indeed the AFC Championship ring, missing a stone.
Dan Marino and Snowflake are both returned to the Miami stadium in time for the Super Bowl halftime show. At the same time, Ace spots the rare bird he had been chasing at the beginning of the movie- only to have it chased away by the opposing teams' mascot (Mark Margolis). Ace and the costumed man get into a fight just as the stadium announcer brings the camera onto Ace Ventura to thank the pet detective for his actions in saving the Dolphins.