Mabel West, daughter of acclaimed mystery writer Raymond West, rebels against his desire for a conventional boarding school education. Driven by her own aspiration to become a detective, she travels to London seeking an apprenticeship with the renowned Belgian detective, Hercule Poirot. Demonstrating her determination, she secures her father's reluctant approval for this path, contingent on periodically visiting her great-aunt, Jane Marple, in St. Mary Mead. This arrangement enables Mabel to learn from both detectives.

As Poirot's junior assistant in London, Mabel participates in criminal investigations, primarily observing and providing minor assistance. During visits to St. Mary Mead, she gains exposure to Miss Marple's distinct investigative methods. Mabel frequently carries her pet duck, Oliver, in a wicker basket. Oliver sometimes emerges during cases, interacting with individuals or, on rare occasions, inadvertently contributing to clue discovery.

Mabel serves as a narrative link between the worlds of Hercule Poirot and Jane Marple. While involved in investigations adapted from Agatha Christie's stories, her dialogue is largely drawn from incidental lines originally belonging to other characters. Consequently, her presence typically does not alter the core plots or solutions of the adapted mysteries. Her character remains consistent as an enthusiastic young observer learning detective methods, without undergoing significant personal transformation. She lives independently in London while working for Poirot and maintains regular contact with both detectives.