Roddy St. James is the pampered pet of a little girl named Tabitha who lives in Kensington in England. The World Cup Final (England vs. Germany) is happening that week. When she and her family go on vacation, Roddy spends the rest of the day doing whatever he wants with only Tabitha’s dolls as his companions.
That night, Roddy hears a strange noise in the house. He goes to the kitchen and investigates it- the noise is Sid- a sewer rat who was shot up into Roddy’s house by a burst water pipe. He completely trashes Roddy’s house. Roddy tries to get him to leave, but Sid, mistaking him for a butler, tells him to do as he says. Roddy then tells him that he should then try out the Jacuzzi. The so-called “Jacuzzi” is really the house’s toilet, and Roddy plans to flush Sid down it. However, Sid, who is not stupid, pushes Roddy in and flushes him down.
Roddy goes on a wild ride down the toilet and into the sewer, where after meeting a few slugs, he finds himself in a rat world similar to that of London, England. Although fascinated by the strange, new world, Roddy desperately wants to get home, and he is told to see the captain of the Jammy Dodger in order to get back home.
When Roddy gets to the boat, the captain of the Jammy Dodger, Rita, a street-smart rat, tells him she has her own problems. Right after that, they are found by two rats- Spike, a small 17-year-old rat trying to be tough and Whitey, a big albino rat who’s not the “fastest tool in the shed.” The two are henchmen of the Toad, a huge toad obsessed with the royal family and freezing his enemies in liquid nitrogen. Spike and Whitey bring Roddy and Rita to the Toad (who wants Rita for “stealing” his ruby, which Rita knows it really is her father’s). After Roddy knocks down some of the Toad’s prized processions, he and Rita are put in the liquid nitrogen freezing machine. Rita and Roddy managed to escape, take the ruby, and Rita pulls out a cable to turn off the power supply in the Toad’s lair. Little does she know the cable is the Master Cable, the cable that will allow the Toad to operate his fiendish plan.
After Rita and Roddy escape and Rita uses the Master Cable as a belt, Roddy follows her to the Jammy Dodger. He tells Rita that her ruby is fake and breaks it to show her. Rita angrily throws things and Roddy and soon after tells him she and her family needed the ruby to “answer our prayers...and now it turns out it was a stupid fake.” Roddy tells her if she gets him back home, he will give her jewels from the jewelry box at his home in Kensington.
Rita stops at her house where her father, mother, more than 34 brothers and sisters, and her grandmother live to get some maps to Kensington. Roddy ends up being invited for dinner. However, after a misunderstanding, Roddy tells Rita the deal is off and leaves on her boat. Rita eventually gets her boat back, stranding Roddy on a rubber duck. With convincing and some singing slugs, Rita agrees that the deal is still on.
The two are then chased by Spike, Whitey, and more of the Toad’s henchmen. They escape once again. Furious, the Toad summons his evil French mercenary cousin, Le Frog, along with Le Frog’s kung-fu hench-frogs. The Toad tells Le Frog to retrieve the Master Cable back to complete his evil plan- destroy all the rats in the city. The reason, the Toad explains, is that he was Prince Charles’s cherished pet until he was replaced by a rat. Le Frog proceeds to go to find Roddy and Rita.
(See the end for the rest of the plot!)