Five English children keep secret their knowledge about a German machine gun and a German fighter pilot

Sixteen-year-old Chas McGill, inveterate collector of war souvenirs, has just found a strange contraption washed up on the beach after a U-boat attack. Is it a detonator? Could it be some sort of tracking device? Chas thinks it might be a radio transmitter that a spy is using to sabotage was shipments in the local harbor - and he's determined to prove it. With the help of his cynical best friend, Cem, the upper-class yet disarmingly frank Sheila, and the tough junior reporter Audrey, Chas manages to uncover real, hard-core evidence of foul play - even murder! Gradually, what began as a "spy vs. spy" adventure turns into something far more serious. The closer they get to the spy, the more reckless Chas becomes. Can he prevent another terrible act of sabotage - without putting his own life in danger?