So: the Cold War meant that spies were terribly interesting people, and Ian Fleming -- who actually had been a spy -- wrote some potboiler novels full of sex and sadism about the sort of spy he would have liked to have been: able to run around, drink infinite amounts of booze, save the world and bed all the good-looking women. This, of course, led into the long-running series of James Bond movies and into imitators and spoofs, like GET SMART and this one, which is not particularly gallant and not particularly funny.
Patrick O'Neal is a newspaper reporter about to the shot by the Red Chinese, probably for being in this movie. Fortunately, he has just been given a dime-store ring by an Ancient Chinese man which will turn him invisible. He has to be naked for it to be effective, of course. He escapes and is promptly recruited by American General Howard St. John to get some vials full of red liquid from billionaire Donald Pleasance -- who looks like the only one enjoying this movie. In this he is aided by Ira von Fürstenberg, and opposed by Henry Silva, who is working for the Chinese. Pleasance has a number of beautiful women working for him, and some thugs. He also has a really weird expression on his face, a Rumanian name, a castle and an Italian accent.
I am pretty sure this is supposed to be funny, because Alberto Lattuada directs, as well as contributing to the script. Maybe it's funny in Italy. From this American's viewpoint, he did a lot better work when he was writing with Fellini.