Enid Blyton 
The Rubadub Mystery 
Barney Mysteries #4

On a summer holiday in the seaside town of Rubadub, Roger Diana, Snubby and Barney witness sinister goings on – explosions at the secret submarine base, suspicious characters at the old inn, lights flashing at night. Suddenly they are plunged right into the middle of another mystery and of course are determined to solve it! Barney finds work in a Pierot show to be near his friends, and begins to realize that this theatrical setting gives him an excellent opportunity to find his long lost actor father – but who is the saboteur at the submarine base, and who can be trusted? It seems that almost everyone at the inn has something to hide!