The Four Men of the title are British WWI veterans who decide to work secretly against enemies of the country. They aren't above a bit of murder or sabotage to serve their ends, but they consider themselves to be true patriots.

The film is a historical drama set during the reign of Elizabeth I (Flora Robson), focusing on the English defeat of the Spanish Armada, whence the title. In 1588, relations between Spain and England are at the breaking point. With the support of Queen Elizabeth I, British sea raiders such as Sir Francis Drake regularly capture Spanish merchantmen bringing gold from the New World.