Nazi doctor Mervyn Johns has escaped from Allied arrest. He murders an English scientist who is about to take over a new position where no one knows him and takes his place. He's working on a vaccine afainst germ warfare, with assistants Nova Pilbeam and Robert Beatty. They think it's humanitarian work. He plans to use it to conquer the world.
It's a rather fanciful thriller, mostly interesting for being Nova Pilbeam's last screen appearance, and for Johns being cast so strongly against type. He was expert at playing small, ground-down men in more than seventy movies, it's sometimes hard to realize that he wasn't what he appeared to be on screen. He was a fine actor, and, given the chance to play this sort of character, did so very well.
As for Miss Pilbeam, her fourteen-year career as a child actor and ingenue were ending at age 29. It seems rather young to retire, but she had had a busy and rather distinguished career. Her second marriage was beginning, and she retired to a thoroughly private life. She died in 2015, age 95.