NOTE: This spoiler sent in by the Saporitos Family.
The film opens in London, with scenes showing the Thames, Big Ben, and Kensington Park. Inside one house, we see a family getting ready to leave on a vacation. As mom and dad grab passports and suitcases, their daughter, Tabitha, goes to feed (over feed) her pet rat, Roddy. Roddy’s cage is two-stories and very plush. (Keep your eyes open for cameos by Gromit, Alex the Lion, and lots of rabbits.)
Once the family is gone, Roddy exits his cage and makes a check list of things to do. He uses a Barbie-type house (complete with occupants) to change his clothes and get ready for the day. He drives around in a toy car, plays golf with frozen peas, and smashes the competition at beach volleyball. In every activity, Roddy is the star. Last on Roddy’s list is Movie Premiere. Dressed in a tux, and pretending that he is 007, Roddy enjoys the film with a blonde (doll) by his side. At the end of the evening, he retires to his cage.
Sometime later, Roddy is woken up by a horrible, grinding noise coming from the kitchen. Grabbing an army man for protection, he heads to the kitchen. Everything seems in order, when suddenly, a burst of crud comes up out of the kitchen sink, and out pops Sid a sewer rat!
Sid can’t believe his luck. He immediately makes himself at home eating food, sitting on the white furniture, and getting dirt and grime everywhere. Roddy tries to make Sid leave, but Sid realizes he’s hit the good life and instead bosses Roddy around. Roddy hits upon a plan, and offers Sid a bath in the whirlpool Jacuzzi (the toilet). That sounds great to Sid, so he follows Roddy, but, as Roddy tries to get Sid into the toilet, Sid pushes Roddy in and flushes him down the drain.
Roddy drops down, down, down into the sewer system. When he lands, he’s horrified that he is covered in sewer water. He stands up in the water and goes to climb out, but is confronted by a slug. Roddy screams. The slug screams. They run away from each other. (Throughout the rest of the film, the slugs provide musical numbers and comic relief.)
Roddy makes his way to wear the rats have created a mini London, complete with Big Ben, street artists, and businesses. Roddy can’t believe his eyes and looks with amazement at all that surrounds him. He meets one rat who swears the end is near because the flood gates won’t hold forever. A police officer confronts Roddy and urges him on his way. A fisherman offers advice, saying that the captain of the Jammy Dodger will be able to get Roddy home.
Upon boarding the Jammy Dodger, Roddy meets Rita, an attractive red-headed rat. Just as Roddy is trying to convince Rita to take him home, another boat shows up. Rita pulls down a screen to conceal her boat, and she tells Roddy to be quiet. By accident, he leans on a rope that blares a horn and reveals the boat to the gangster rats that are searching for Rita. Rita and Roddy are held captive by Whitey, a muscular former lab rat, while Spike questions Rita about the ruby she stole from Toad. She refuses to return it, claiming her father found it. When that does not set her free, she claims that Roddy is an international jewel thief and that he has it. Roddy notices that Rita’s back pocket is bulging, and he points out that the ruby is in her back pocket. Rita and Roddy are taken to see Toad.
Toad takes the ruby and scolds Rita for having stolen it. Upon learning that Roddy is from up above, Toad shows him a collection of tacky knick-knacks that represent members of the Royal family. After Roddy accidentally breaks every item, Toad has them dragged to the freezer, where his henchmen will use liquid nitrogen to imprison Rita and Roddy. Using a paperclip, Rita and Roddy are able to escape (freezing Whitey and Spike in the process). As they run away, Rita not only steals the ruby, she also swipes a cable from the sanitation grid.
As a frozen Whitey and Spike pursue Rita and Roddy, we see Toad talk to a group of tadpoles in a blender jar. He is planning to flood the rats out during halftime of the World Cup, but he needs the master cable that Rita stole to do it. When Whitey and Spike return without the cable, Toad send them back out to try again. He also contacts his cousin, Le Frog for help.
We see Roddy and Rita on the Jammy Dodger again, and as he holds the ruby, he tells her it’s a fake. She doesn’t believe him, so he smashes the ruby on the railing of the boat, breaking it into tiny pieces. Rita is crushed, that ruby would have changed everything for her family. So, Roddy makes her a deal, if she takes him home, he’ll give her a real ruby out of the jewelry box in his house. Rita spits in her hand to close the deal and eventually so does Roddy. They head to Rita’s parents house to pick up her dad’s maps of the sewer system.
Rita is welcomed enthusiastically by her parents, grandmother, and siblings (there are MANY!). Roddy watches from outside, but then he is invited in to meet the family. Rita’s grandmother thinks that Roddy is Tom Jones, and she is very excited. Roddy meets one of Rita’s little brothers’ who introduces himself as “Shocky” right before he uses battery cables to shock Roddy. Rita gets the maps from her father, and they are just about to head out, when another brother suggests that they turn Roddy into the Toad for the reward being offered. Roddy overhears, and thinks Rita’s going to turn him in, so he runs outside and steals the Jammy Dodger. At the same time, Rita is reminding her brother that when you make a deal, you stick to it. However, she is not pleased that Roddy stole her boat.
Roddy runs out of gas and is wondering what to do, when Rita shows up, and kicks him off the boat. He is floating on a rubber duckie, while Rita refills the gas tank. She starts the boat and heads out, Roddy is able to grab a rope attached to the boat and tie it around the duckie’s neck. As Rita drives away, Roddy is towed behind the Jammy Dodger singing (with the slugs chiming in).
Just as Rita lets Roddy back on the boat, Whitey and Spike (plus some other henchmen) show up. Rita quickly tries to out run them, but they are gaining. She manages to allude several of them, but Whitey and Spike are gaining. Roddy has Rita swerve close to a huge balloon so he can grab the string. Then, she drives through a tunnel and the balloon blocks the other rats from chasing them. However, Spike has a pointy weapon, and he manages to pop the balloon, so he and Whitey can continue chasing them. Spike launches himself onto the Jammy Dodger using a mouse trap, however, before he can get the master cable, the spring he’s on reaches its maximum extension, and he is pulled off the boat.
That evening, Rita and Roddy talk over dinner. Rita comments that she would love to meet his family, which gives Roddy an uncomfortable moment, because he doesn’t have a family like hers.
When they wake up in the morning, they are swarmed by ninja frogs henchmen of Le Frog’s. Via cell phone, Toad tells Rita that he’s won. The tables are quickly turned, however when Rita throws a fly into the air and all of the ninja’s try to catch it. With their tongues tangled together, Rita hooks a branch into the knot, and they are lifted off the boat.
Le Frog manages to get the master cable (Rita has been using it as a belt), just as the Jammy Dodger goes over a waterfall (inside the sewer). Rita and Roddy manage to chew through the ropes that hold the emergency parachute to the boat, and escape through the updraft of the waterfall, and the parachute lifts them up and out of the sewer system. Floating in the air, Rita and Roddy are able to find his house and enter through the chimney.
Roddy give Rita two jewels, one ruby for her family and an emerald for a Jammy Dodger II. She then asks for a tour of the house. While Roddy shows her his home (cage), Rita realizes that he is all alone. At that moment, in walks Sid. Frantically, Roddy pretends that Sid is his brother, Rupert. When, Rita and Sid greet each other having met down in the sewer, Roddy defends his home, pointing out all of the things that he has. Rita, saddened by his defensiveness, leaves to head back to the sewer (she is flushed by Sid).
Meanwhile, back in Toad’s lair, Le Frog delivers the master cable. Toad cackles with glee and reveals his plot to flood the rats out during halftime of the World Cup Soccer game (when everyone goes to the bathroom and flushes at the same time). The master cable will allow Toad to raise the flood gates. In addition, Toad then plans to repopulate the area with his offspring, and he opens the refrigerator door to show Le Frog hundreds of tadpoles.
Sad without Rita, Roddy sits on the couch (surrounded by the mess Sid has made). As Sid returns, Roddy has a brain flash, and realizes what Toad’s plan is. Dragging Sid to the bathroom, Roddy makes him promise to be nice to Tabitha (the little girl who owns him) and then he has himself flushed so he can save Rita and the town.
All of the rats have gathered in the center of town to watch the World Cup Final. English is winning! As Roddy arrives, he sees Rita bound and gagged, hanging in front of the sewer drain, whose cover is now open. Grabbing a balloon and a small fan, Roddy flies up to save Rita. However, Le Frog and Toad try to stop him. As Toad walks across a pipe, it cracks and a burst of liquid nitrogen freezes his foot in place.
At this moment, its half time, and Roddy can hear the water coming.
Roddy then tricks Toad into sticking his tongue in a gear shift, which pulls Toads tongue farther and farther in. Le Frog ends up stuck to Toad as the two of them are slowly pulled towards the gears. This breaks the pipe, which swings down into the path of the oncoming water, freezing it with the liquid nitrogen. The town is saved and Roddy is a hero!
Roddy and Rita build the Jammy Dodger II and set off on their next adventure.
The film ends with Tabitha and her family returning home. As she enters, she calls for Roddy and tells him that she brought him a present - a cat!