1865, The American Civil War: Five POW manage to escape by balloon from Fort Richmond. Caught in a storm and with the balloon damaged they find themselves 'shipwrecked' on a volcanic desert island somewhere in the Pacific. Just before crash landing their leader, the brilliant engineer Cyrus Smith falls to his apparent death in the raging waters, but later turns up completely unharmed and with no memory of how he was saved. His fellow castaways are: the famous journalist Gedeon Spilett, the salty sailor Bonaventure Pencroff, the orphaned boy Harbert Brown and the black servant of Smith, Nab. The dog Top also joined them in the escape.

The war in Europe is ending, but the American troops have to find a scientist on the run who is also chased by some surviving SS forces.