An evil scientist (Howard Vernon) creates a murderous, invisible ape-man.
Another reviewer said this was a standard Jess Franco film, by someone other than Franco. Yep, that pretty much sums it up -- we even have Howard Vernon as Dr. Orloff, a role he has filled well in the past, and the poor quality filming of people being tortured (by an unseen force).
I actually rather liked this one, and would say it exceeded the average Franco attempt. The special effects are a bit cheesy, but not bad for the time it was released and the presumably low budget. I am not sure if I could have done any better myself.
Now I want to learn more about Pierre Chevalier... he might have been a hidden gem. I will have to track down "Human Cargo" or "Panther Squad".