20 Mensou ni Onegai!! is a about a nine-year-old boy named Ijuuin Akira who steals beautiful and valuable objects to please his two mothers and is known to the public as the dashing, clever thief named the Man of 20 Faces.

By day Ijuuin Akira is a normal student at Clamp School, but at night he becomes the mysterious thief 20 Masks. His thefts are usually subject to the whims of his two stepmothers, which Akira pulls off without any true objections or interactions. One night Akira ducks into the room of 5-year-old Ookawa Utako in an attempt to hide from the police. Although the pair shares a four-year age difference, they find themselves quickly falling for each other even as Akira finds himself being increasingly more involved with thefts, some of which include Utako's family. This as well as several other obstacles give the young couple's budding relationship a fair amount of difficulty.