Colditz castle was used by the Nazis to hold the "bad boys", (those who regularly tried to escape from other camps). At all times the guards outnumbered the prisoners and, because some political prisoners were also held there they were *very* strictly monitored. But if you put all those people in one place and they're all trying to escape, well ...

On the same day two boys are born: the pauper Tom and prince Edward. As a kid, Tom sneaks into the palace garden and meets the prince. They change clothes with each other but the guards discover them and throw out the prince, since they are almost identical. Nobody believe them when they try to tell the truth. Soon after, the old king dies and the prince will inherit the throne. The evil earl of Hertford attempts to murder the prince to gain power for himself.

All leave is canceled so that a British submarine can be sent after a new German warship. They chase it so far that they have no fuel to get home.