The Mystery of the Burnt Cottage

Who could have set fire to Mr Hick's cottage? The suspects include a tramp and even the housekeeper, Mrs Minns. The Five Find-Outers and Dog aim to solve the mystery before Mr Goon, the grumpy policeman.

This book kicks off the series and introduces the main characters along with a bunch of others.
The story starts with Larry and Daisy Daykin spotting a flare up out the window at about half past nine one evening. They get dressed and sneak out of the house with Larry explaining (a little vaguely) that "Mummy and Daddy are busy, so they won't know anything about the fire." Vagueness about unnecessary details is Enid Blyton's style throughout this series. Parents never get in the way of the plot.