During the Nazi occupation of a Czech city civilians are being rounded up on the slightest of pretexts and shot. One day three high school boys who crack jokes about a recently deceased "hero of the Reich" are pulled out of school by the Gestapo.

An occupation drama about a group of Czech scientists who, in a remote factory, investigate how to produce the vital penicillin as easily as possible - but for a long time they have been unable to find the only right mould to use as a base.