Berlioz has blue eyes, a small and skinny black furred body with a lighter grey stomach. He wears a small red ribbon around his neck. He is named after french composer Hector Berlioz.

Being the pet of retired opera singer Madame Adelaide, Beriloz and his family live a very comfortable and luxurious life. One day, Madame Adelaide announces that she wants to pass her inheritance on to her four cats, and for Madame Adelaide's butler Edgar to look after the cats until they die and then eventually inherit the money for himself.

Edgar believes that he will be dead before he has the chance to inherit the fortune, and tries to get rid of Dutchess and her kittens. He drugs the cats with sleeping pills, and then goes to the countryside to get rid of them. The cats are then left in the countryside, and so they are made to fend for themselves. While being there, Duchess meets an alley cat named Thomas O'Malley, who offers to guide her and the kittens back to their home in Paris.

On their return, the cats hitch a ride on the back of a milk cart but are then chased off by the driver. Marie subsequently falls into a river and is saved by O'Malley. O'Malley and Dutchess then fall in love, and Duchess is asked to marry Thomas. Dutchess turns Thomas down out of loyalty to Madame Adelaide and her home. The cats then return to the mansion with the help of O'Malley, where Dutchess and the kittens have to say goodbye.

Upon their return however, Edgar re-captures the cats and this time decides to ship them off to Timbuktu. O'Malley returns and saves Dutchess and the kittens, with the help of other alley cats. In the end, Edgar is instead locked into the trunk, and sent to Timbuktu instead. Madame Adelaide then re-writes her will, excluding Edgar and including O'Malley. O'Malley then marries Dutchess, and now lives with her and the kittens in Madame Adelaide's mansion.