Superman - Destruction, Inc

Destruction Inc. (1942)

In Metropolis, the night watchman from the Munitions Works is found dead in a swamp. Clark Kent and Lois Lane both want to cover the story. Lois goes undercover as a factory worker and overhears a plan to blow up the factory. She is caught and put in a test torpedo with dynamite. Clark, who was posing as the night guard, frees himself and turns into Superman to save Lois. They stop the plan to blow up the factory and arrest the culprits. Lois reveals that she knew Clark was the night guard all along.