This is the second movie based on Dashiell Hammett's quintessential detective novel The Maltese Falcon, and the least faithful. Obviously the 1941 version directed by John Huston and starring Humphrey Bogart and Mary Astor is the most famous, but there was also a 1931 adaptation with the novel's original title as well. This version ditches the title, the character names, the tone and many of the plot details, along with the titular object itself. It's changed from a falcon into a medieval French artifact, although it's still little more than an excuse to move the plot along.
That plot, with Warren William as a caddish private eye (here named Ted Shane rather than Sam Spade) hired by Bette Davis' femme fatale, is horribly convoluted and not really important until the end, when it all gets dumped out in a rush. Meanwhile, the movie is really about Shane's pathological womanizing (he calls every female character "kitten" or "precious") and cavalier attitude toward the law and personal safety, which make him seem more creepy than endearing. William's performance is extremely grating, and the movie is going for some sort of breezy comedic tone that doesn't work at all. Shane's ditzy secretary gets more screen time than Davis does as the ostensible second main character, and Davis is about the only one who doesn't mug for laughs.
Which means she pretty much gives the only good performance in the movie, vamping where appropriate and showing the menace that all the other buffoonish criminal characters can't muster. Still, she reportedly hated doing the movie and doesn't exactly give it her all; it's just that the rest of the movie is so silly and frantic, she's the only who seems like she has any idea what she's doing.