A Fish Called Wanda begins with the London gangster George Thomason and his crony Ken Pile planning a jewel heist. They hire two Americans: a con woman named Wanda Gershwitz, and weapons expert named Otto West to assist with the plan.
Ken has a terrible stutter, and loves animals which the mean-spirited Otto takes advantage of throughout the film. While Otto and Wanda are lovers, they keep this a secret from George and Ken. Instead, they pretend to be brother and sister so Wanda can seduce the men to get what she wants from them.
The heist goes off without fail, and as the thieves flee, they nearly run over an elderly woman out on a walk with her doggies. She catches a glimpse of George without his ski mask on. George, Wanda and Otto hide the diamonds in a small safe at an old garage. They then split up with plans to lay low for a few days before sneaking out of the country. However, once Otto and Wanda leave, George moves the diamonds (which Wanda and Otto will later find out are gone) to an undisclosed location.
Back at Otto’s, he tries to make sweet, sweet love to Wanda, but she instead has him phone the police with George’s name. The cops arrest George for the heist when they find incriminating evidence on his pants from the jeweler’s.
The police allow George to meet with Ken, and George slips a small key to him which he later hides in the tiny treasure chest of his aquarium. Wanda spots Ken hiding it, and takes the key once Ken’s out of the room. She hides it in her locket for safekeeping.
A police lineup spells the end for out heroes
Later at a police lineup, the old lady George nearly killed while fleeing the crime scene IDs George. At a grand jury hearing, George gives Ken a sly nod; marking her for murder.
Wanda tries to learn the location of the safe deposit box, so she seduces George’s barrister, Archie Leach to find out more information about George’s case. Archie’s in a loveless marriage, so he easily falls for Wanda’s love trap.
However, Otto begins interfering with Wanda’s seduction of Archie out of jealousy. He threatens Archie several times, even going so far as to dangle him out a window. Furious, Wanda explains to Otto that his behavior could ruin any chance they have to find out where George hid the diamonds. She forces Otto to apologize to Archie.
Archie and Wanda meet up many times, and after one rendezvous at Archie’s house, Wanda accidentally leaves her locket there. Archie’s wife, Wendy, returns home with her daughter after her tire gets a flat. She finds the locket, which she believes is a gift from Archie. Wanda demands Archie return the locket, so Archie stages a robbery of his own to retrieve it.
As Archie is stealing from his house, Otto arrives to apologize, but he thinks Archie is a real burger. Otto ties him, and beats him. Realizing his mistake, Otto leaves Archie tied up and flees the home just as Archie’s wife returns.
Archie hides the locket in his mouth until he hands are free, then spits it out,. He hides it in his pocket. As soon as he can, he rushes off to a friend’s apartment to meet Wanda where he gives her the locket.
A trip to pound town
Archie then takes Wanda to pound town, where he learns Wanda gets turned on when he speaks a foreign language. A family that rented the apartment for the day interrupts the lovemaking session. Naked and embarrassed, Archie ends their affair.
Archie returns home where Otto accosts him. He then apologizes to Archie for his actions, and gives him permission to continue his affair with Wanda. Unfortunately for Archie, Wendy overhears the whole conversation from an open window above them.
Meanwhile, Ken is on his mission to kill the old woman. Unfortunately, Ken misses her, and accidentally kills her three dogs instead. Being the great animal lover he is, this causes him much distress. However, the deaths of the old’s dogs causes does the trick. She keels over with a heart attack from the stress of losing her doggies.
Ken tells George the old woman is dead, and George orders him to retrieve the diamonds. He wants him to pack for Rio de Janeiro; saying that they will take Wanda with them, but kill Otto later.
Otto at this time, learns Ken knows the location of the diamonds. He finds Ken, ties him up, and shoves ketchup-covered French Fries in his nose. Otto then eats Ken’s pet fish one by one until only his beloved angelfish, Wanda, remains. He eats that one too. Ken sputters out that the diamonds are in a safety deposit box at a hotel near Heathrow Airport.
Of course, the key is not in the aquarium, so Otto calls Wanda. She makes a deal with Otto that she’ll give him the key and diamonds if they can flee to Rio together once she testifies on George’s behalf.
Betrayal in the courtroom
Later in court, Wanda takes the stand. When Archie questions her about her relationship with George, she begins to portray him as an honest man, but then she give a few details that implicate George in the crime. She says that George left their apartment armed with a shotgun right before the crime. This revelation stuns Archie, and he accidentally calls her darling which infuriates Wendy who is watching above from the mezzanine.
Wanda’s comments enrage George, and he attacks her. The guards jump to action to subdue him. In the melee, George hits Archie, and knocks him down. As Archie sits alone in the cleared-out courtroom, Wendy walks up, slaps him, and tells him she’s divorcing him.
Archie then talks with George privately, and assures him that he will coach Wanda on the witness stand so that George’s exonerated. The two strike a deal for George to receive a lesser sentence, and he asks George to reveal the location of the diamonds. George tells Archie to find Ken because he knows where they are.
As Archie leave the courthouse, he spots Wanda sneaking off, and he takes her with him to Ken’s place. Archie goes in alone after Ken. As he finds Ken tied up, he hears a car crash outside. Otto found Wanda alone, and he took off with her to the airport. Archie tries to get Ken to tell him where the diamonds are, but he’s too flummoxed to spit out the words. So, Archie makes Ken write down the name of the hotel, and they head out for the diamonds.
A race for the diamonds
Wanda and Otto retrieve the diamonds, and then head next door to the airport get their tickets to Rio. As they walk up to the gate, Wanda clubs Otto over the head, and then locks him in a small closet.
Archie arrives to find Otto stumbling around in a daze. He grabs Otto’s gun, but Otto easily tricks Archie into dropping it. Otto then grabs it again, and decides to humiliate Archie one last time.
He forces Archie to stand in a barrel of used oil outside the terminal, and then insults him and all his Britishness. As he prepares to shoot Archie, he spots Ken driving towards him on a slow-moving steamroller, and taunts him one more time too. However, Otto steps into some wet concrete and becomes stuck.
Otto apologizes to Ken for eating his fish, but Ken continues on. The steamroller squishes him into the fresh cement, and continues rolling on. Ken becomes so overjoyed at his revenge, that he cures his stutter. Unfortunately, the police immediately arrest him.
Archie makes it to Wanda’s plane, and the two reconcile. As the plane takes off, Otto’s face pops up in their window, but the moving plane shoves him off. Closing titles reveals that Archie married Wanda, and had children. The two founded a leper colony soon after. Ken became a Master of Ceremonies at London’s Sea World, and Otto became a Minister of Justice in South Africa for a happy ending for all.