John Watt is supposed to be looking after Mister Nibbles, the biology lab hamster. When he escapes and is mangled by John's cat, he thinks he's in big trouble.
But when Bright finds Doctor Frankenstein's notebook and puts Mister Nibbles back together again (with a few bits of rat, gerbil, and guinea pig mixed in) and brings him back to life, John realizes that his troubles are just beginning. And that's before the reanimated Mister Nibbles starts to grow...