It is set in Des Moines, Iowa—no, just kidding, San Francisco—in the months immediately before the 1906 earthquake.
Blackie Norton (Gable) is a saloon keeper on the Barbary Coast, then the main red light district of the city. Into his saloon wanders Mary Blake (MacDonald), a new arrival to the city and would-be opera singer who has failed to find a job and is desperate for any employment. Blackie hires her for $75 a week as a dance hall singer. Initially, Mary resists Blackie's crude sexual advances, but soon they fall in love.
Meanwhile, Blackie has been recruited by fellow Barbary Coast saloon owners to run for the San Francisco Board of Supervisors on a platform of improving the fire codes and safety standards of buildings on the Coast. In this, he is opposed by Jack Burley, a Nob Hill aristocrat and Barbary Coast slumlord who doesn't want to pay to upgrade his properties. Burley is also fighting Jack for Mary, both professionally—he is a patron of the San Francisco Opera and wants Mary to sing there—and personally, as he wants Mary to be his wife. Blackie for his part wants Mary all to himself. Blackie's old friend Tim (Tracy), now a Roman Catholic priest who works in a Barbary Coast mission, is also against his relationship with Mary, believing her to be too fine and noble to be a dance hall girl and a saloon keeper's lover.
All of these plot elements are short-circuited when the whole city is destroyed on April 18, 1906.
The only film to pair fellow MGM stars Gable and MacDonald, partly because Gable couldn't sing, and partly because they loathed each other. note (Legend has it that Gable deliberately ate a spaghetti lunch loaded up with garlic before filming a kissing scene with MacDonald, to make his breath particularly bad.) The song "San Francisco", composed for this film and performed several times, has become an iconic theme tune of the city.
Apparently, D. W. Griffith, who had not directed a movie in five years and in fact would never direct another film, was visiting the set one day, leading his old friend and former protege Woody Van Dyke to let him direct the earthquake scenes.
- Answer Cut: A big showdown between Blackie and Burley ends with Blackie demanding that Mary picks between the opera and him. The movie cuts to a poster advertising Mary's act at Blackie's club.
- Artistic License History: Really, would any dancehall variety show have been likely to still be going at 5 o'clock in the morning?
- At the Opera Tonight: Two different scenes have Mary singing opera at the Tivoli Theater.
- Belief Makes You Stupid: Part of Blackie's Straw Atheist deal, as he mocks the poor schmucks who come to Tim's church asking God for help instead of taking what they want.
- Disaster Movie: One of the first, but not the first, as the genre dates at least as far back as 1933 and Deluge. Also atypical of what would become the standard format, as the earthquake doesn't hit until there are only 20 minutes left in a nearly two-hour movie. MGM spared no expense, putting entire sets on raised platforms and shaking them to simulate an earthquake. The effect still looks good in the 21st century.
- Driving a Desk: Couldn't they have mounted a camera on the front of the carriage when Blackie and Mary are tooling around?
- Foreshadowing: The opening scene has a building on the Barbary Coast burn down, leading to much chatter about the lack of fire safety in the district and causing Blackie to run for office.
- Fruit Cart: A variant of this trope proves surprisingly effective tragedy, as a toppling statue crushes a fruit vendor and his cart.
- Getting Crap Past the Radar: Two suspiciously well-dressed, good-looking young women with sad expressions on their faces are in the seats for Reverend Tim's otherwise sparsely attended sermon. Apparently, they're prostitutes.
- Hollywood Atheist: Blackie pours scorn on Father Tim's faith, and the habit of poor people of asking God for help instead of helping themselves. This is all so he can have a moment of religious conversion at the end of the film and finally be worthy of Mary's love.
- Melismatic Vocals: "Sa-a-a-a-a-n Francisco, open your golden gate..."
- New Year Has Come: The movie opens with the good folks of the Barbary Coast celebrating New Year's 1905-6.
- Red Light District: The Barbary Coast had been the red light district of the city since it was founded during the gold rush, teeming with dance halls and prostitutes and gambling dens. Blackie wants to build the Coast up. In Real Life the city cracked down on vice and corruption in the Coast just a few years after the quake, causing such activities to move to the Tenderloin.
- Romantic False Lead: Burley. Mary clearly loves Blackie but winds up accepting Burley's proposal after she and Blackie break up. It never occurs to her that she could just be single and work in the opera for a while.
- Tempting Fate: A drunk spectator at the Chicken Ball really takes this trope to the limit when he says "Eat, drink, and be merry, for tomorrow you die."
- Third-Person Person: Blackie has a habit of slipping into this when he gets bossy and controlling. ("That's the way you're going to sing it, or you're not going to sing it for Blackie.")
- Worst News Judgment Ever: The paper on April 17, 1906 has a front-page headline announcing Mary and Burley's engagement. Presumably, the next edition had bigger news to report.