The film opens in Russia during another one of its starvation-condition Five Year Plans. A trio of Russian delegates (Sig Ruman, Felix Bressart and Alexander Granach) are sent to Paris to sell the Imperial Jewels for ready cash. Grand Duchess Swana (Ina Claire), who once owned the jewels, sends her boyfriend Count Leon (Melvyn Douglas) to retrieve the diamonds (the duchess lacks the moolah to buy them herself). Leon pulls strings to prevent the three Russians from making their sale, and to ensure that they won't give him any more trouble, he turns the trio into full-fledged capitalists, wining and dining them all through Paris.

Moscow then dispatches the humorless, doggedly loyal Comrade Nintotchka (Garbo) to retrieve both the prodigal Soviets and the gems. When Leon turns his charm on Ninotchka, she regards him coldly, informing him that love is merely a "chemical reaction." Even his kisses fail to weaken her resolve. Leon finally wins her over by taking an accidental fall in a fancy restaurant, whereupon Ninotchka laughs hysterically for the first time in her life. She goes on a shopping spree and gets drunk, while Leon begins falling in love with her in earnest. Grand Duchess Swana manages to swipe back her jewels while Ninotchka sleeps, then offers to return the jewels if Ninotchka will go back to Moscow and leave Leon for good.

Disillusioned by Swana, Leon tries in vain to follow Ninotchka to Russia. The lovers are at last reunited when Ninotchka is sent to Paris once more by commissar Bela Lugosi to straighten out the three Soviets who'd originally accompanied the jewels, and who are now successful restaurateurs. As a bonus to the frothy scriptwork (by Billy Wilder, Charles Brackett and Walter Reisch) and surefire star power, Ninotchka includes what is perhaps Bela Lugosi's most likeable and relaxed performance.