This isn't really a movie. It was really a pilot for a television show in 1965, but it never got picked up, so they released it as a feature. And I'm glad they did, because it's a lot of fun.
It stars Leslie Nielsen as a kind of supernatural detective who sometimes has to help the San Francisco police with their more... eldritch cases. He has an apartment full of cool-looking things, a dwarf manservant named Nikola, and a secret room that looks like it has a fancy scientific lab in it. Nielsen is a lot more fun here than he would become later in his career when he's playing deadpan stiffs in wacky comedies.
The plot is kind of complicated and involves evil demons and gods and stuff. Oh, and Werner Klemperer is in it, but he's using a British accent and he's buried under makeup, so he's not very Colonel-Klinkish. There's just a touch of Lovecraft. I wish they'd made this a series, because it would have made a fun counterpoint to Wild, Wild West.
Also, now I have to go learn more about Leslie Nielsen's "serious" career, because I had no idea he was so enjoyable.