In Agatha Christie's 'Third Girl,' the renowned detective Hercule Poirot finds himself entangled in a perplexing case when a young woman, Norma Restarick, approaches him convinced she has committed a murder, only to flee before providing details. Her sudden disappearance sparks Poirot's investigation into her life, uncovering a complex web of deceit, troubled family relationships, and the mysterious identity of a 'third girl' living with Norma and her flatmates. Poirot must navigate unreliable testimonies and hidden truths to unravel the unsettling mystery before more harm comes to Norma or others connected to the enigmatic events.