Ahead of the imminent invasion of the Channel Islands by Germany, most of the inhabitants choose to evacuate, but a few families decide to stay put. Teenager Terry Benson recruits his friends Carol and Cliff Delamere to form a resistance group with the help of a radio set.
Major Heinrich Wolf commandeers at room at the Delameres' house, much to the dismay of Cliff, Carol and Terry. Terry's father Tom is forced to go into hiding.
Major Wolf discovers the gang's radio set but not before Terry receives news of a secret drop from England.
Major Wolf arrests Jean Delamere for aiding Tom Benson in his escape - but changes his mind when Jean demands he takes on responsibility for looking after the children. Terry fixes the radio and is instructed to help a secret agent sneak onto the island.
The children's plans are discovered by Jean - and their neighbour, Mr Prout, who has a surprise of his own.
When Cliff's prank goes wrong, Major Wolf bars them from playing at the mill, unaware that it is their secret base.
The children disobey the Major's orders and he catches them in the Mill. The Major threatens to send the children to France. The adults form a plan to help the children to escape, but time is running out...