Flash Gordon’s best friend is sent to explore suspicious conditions on a strange planet, but returns – brainwashed – and tries to murder Flash! The attempt fails.d Flash and Dale decide to find out who’s doing the brainwashing. They learn the truth in this clip – as they come face to face with Akim the Terrible. Dale (Irene Champlin) is placed in a weird device, her forehead strapped down, and interesting reactions unfold on her face as the machine does whatever it is it’s doing. Then it’s Flash’s turn. From the 1950’s series.
Note: Apparently some frames are missing in the clip, at any rate it looks a bit like stop motion.
Source: Internet Archive.