Colby kept the Crown Jewels of England safe from anyone, including the Canterville Cat, trying to steal it. Monty believes that his ancestor scared the cat, but it was actually the cat who scared Colby. Ever since then, Colby was cursed to haunt the tower.
While the Rescue Rangers go to the Tower of London to stop Fat Cat from stealing the Crown Jewels of England, Monty sees a painting of Sir Colby and tells his friends about him and how he scared the Canterville Cat from stealing the Crown Jewels. When Colby's ghost comes out of the painting and starts crying after failing to scare the Rescue Rangers, he tells them that he can't scare anyone not even the Canterville Cat that actually caused him to desert his post and that it was his cowardice and shame that led him to being cursed to haunt the Tower ever since then and the only way to break the curse is for a descendant like Monteray Jack to do a brave deed in his name. They then run into the ghost of the Canterville Cat, who is actually Fat Cat's Gang in disguise.
After escaping the cat, the rangers and Colby get to the room where the Crown Jewels are. They then run into the Canterville Cat again. Fat Cat traps Chip, Dale, and Gadget while Monty runs away. When Colby mentions leaving his friends behind due to his fear, Monty goes back to rescue his friends.
While Monty rescues his friends after defeating the "Canterville Cat's ghost" in Colby's name, Colby defeats Fat Cat by scaring him out of the Tower. The curse is broken and Colby leaves the Tower and goes to Heaven as he bids the Rescue Rangers farewell.