Based on Al Jolson's 1931 Broadway hit, Wonder Bar transposes the Grand Hotel formula to a lavish nightclub in Paris' Montmartre district. Presiding over the evening's entertainment is manager-emcee Al Wonder (Jolson), who after greeting his guests in a multitude of languages (a la Joel Grey in Cabaret) introduces a steady stream of top variety acts.
The star attraction of the Wonder Bar floor show is the Latin dance team of Inez (Dolores Del Rio) and Harry (Ricardo Cortez). Al worships Inez from afar, but she is hopelessly in love with Harry, a no-good louse who is carrying on with Liane (Kay Francis), the wife of prominent banker Renaud (Henry Kolker).
Meanwhile, German military officer Captain Von Ferring (Robert H. Barrat), who has lost his fortune to bad investments, enjoys one last fling at the Wonder Bar before committing suicide.
The two main subplots converge when Inez stabs Harry out of pique, whereupon the ever-loyal Al deposits Harry's body in Von Ferring's car, knowing full well that Von Ferring intends to drive himself off a steep hill to his death.
Never letting Inez find out that she killed Harry, Al stands stoically aside as she finds true happiness with composer Tommy (Dick Powell).
Lest this all sound heavily somberly serious, it should be noted that Wonder Bar is chock full of laughs, from both Jolson (who runs through quite a repertoire of tried-and-true routines) and the drunken antics of tired business men Hugh Herbert and Hobart Cavanaugh.
The musical numbers staged by Busby Berkeley range from sedate to incredible, with the bizarrely racist 10-minute Goin' to Heaven on a Mule (truly a jaw-dropping experience) falling into the latter category.
The film's most outrageous moment, however, is an uninhibited chunk of homosexual humor on the dance floor (Boys will be boys!, crows Jolson) which just barely squeaked past the Hollywood censors!