If you're looking for plot you won't find much beyond a scientist blowing up his head, shrinking it and then a lab assistant blowing it up after tinkering too much. It's not much but it's amusing because it may be one of the first uses of multiple exposure.
It's amusing and fun experiment in special effects. They took an idea and went with it and the result is humorous and a significant part of special effect history.
Review of L'homme à la tête en caoutchouc
