The team go underground (literally) in a daring plan to strike at the heart of Germany at the very start of the Second World War.

No sooner has Biggles listened to the declaration of war on the radio than he is called into see Raymond who has devised a plan just for him - it involves making use of a small rock called Bergen Ait, right in the middle of the Baltic. The rock, complete with the 4 aircraft and accompanying stores hidden in a cave in its interior, becomes the base for Z Squadron.

From this secret base the team launch several daring raids against Germany. But when they stumble over a secret depot, and a missing liner, that the fun really starts. Biggles survives two firing squads, being torpedoed, and a long swim in the Baltic wearing only his underwear. Algy manages to land on the rock rather than beside it. And Ginger, as usual, manages to get lost.

In the end the base is destroyed by Biggles and the team escape to England in a submarine.