The Man in the Brown Suit by Agatha Christie

Adventure-seeking Anne Beddingfeld is in London when she sees a stranger fall to his electrifying death in the tubes. A dreadful accident? If so, who is the man in the brown suit fleeing from the scene? Curiosity, and one cryptic clue, lead Anne aboard a cruise ship to Cape Town and into the confidence of Colonel Race, counterintelligence officer for MI5. Drawn into a dangerous conspiracy, Anne’s found the adventure she wanted. And as she’s chased across continents, all she must do now is survive it.