Philip MacDonald

''Warrant for X'' is widely considered one of the finest entries in MacDonald's popular Colonel Anthony Gethryn detective series. Sheldon Garrett, an American playwright visiting London, accidentally overhears a conversation in a dimly lit tea shop between two people he cannot see. Garrett realizes they are meticulously planning a horrific crime involving a child, but he has no names, no faces, and no location.
The London police dismiss his story due to the complete lack of actionable evidence. Garrett connects with amateur detective Colonel Anthony Gethryn, who takes the threat seriously. Gethryn builds an intricate case from a few minimal items, including a shopping list and a London Underground timetable.