The Purple Death

In 1944, the story of Captain America was serialized for theaters. It’’s quite different from the Avenger you know today. From Wikipedia:

Captain America is a 1944 Republic black-and-white serial film loosely based on the Timely Comics (today known as Marvel Comics) character Captain America. It was the last Republic serial made about a superhero. It also has the distinction of being the most expensive serial that Republic ever made. It also stands as the first theatrical release connected to a Marvel character; the next theatrical release featuring a Marvel hero would not occur for more than 40 years.

The serial sees Captain America, really District Attorney Grant Gardner, trying to thwart the plans of The Scarab, really museum curator Dr. Cyrus Maldor - especially regarding his attempts to acquire the "Dynamic Vibrator" and "Electronic Firebolt", devices that could be used as super-weapons.

In a rare plot element for Republic, the secret identity of the villain is known to the audience from the beginning, if not to the characters in the serial. The studio's usual approach was the use of a mystery villain who was only unmasked as one of the other supporting characters in the final chapter.

The series is archived at YouTube if you want to watch them all. There are 15 episodes, with bonus material. (via Metafilter)