The Whisper in the Gloom - Nicholas Blake - Bog

A small boy playing in the park is handed a crumpled piece of paper by a stranger, who then collapses and dies. The boy, realising that he himself is now in danger, flees from the park with the help of detective Nigel Strangeways, only to discover that the mysterious message consists of just his own name and age: Bert Hale 12.