During a solar eclipse, Seymour Krelborn (Rick Moranis) discovers a strange little plant while walking around Chinatown. He brings this little flytrap back to Musknik's flower shop in skid row where he works and tries to nurse it to health. He even names the little bud after the girl he has a crush on, Mushnik's shop girl Audrey (Ellen Greene). But Seymour soon discovers after cutting his finger that the only food little Audrey II (voiced by Levi Stubbs) takes to is human blood. And miraculously, the little plant speaks - only to Seymour - and asks him: "Feed me!"

Just as Audrey II is beginning to blossom, so is Seymour's relationship with the real Audrey. She's dating a lunkhead dentist who treats her badly, as can clearly be seen by the black eye she sports one morning. And since Seymour can't keep going to the butcher shop Audrey II's food, the plant convinces Seymour that they can kill two birds with one stone: knock off the dentist and let Audrey II dispose of the evidence, ridding Audrey of the abusive boyfriend and giving the plant some well-needed food. In other words: "The guy sure looks like plant food to me!" The plan is more or less accomplished, but guilt immediately overtakes Seymour. Audrey II, however, is overtaken by something much worse: bloodlust.

The dentist's body has accelerated the plant's growth as well as its appretite. A stronger and more aggressive Audrey II now demands more bodies, and Seymour is too scared to protest, worried that the next meal might be him or - even worse - Audrey. Meanwhile, the huge plant has attracted a crowd and media attention. Seymour doesn't want to feed Audrey II anymore, but Mr. Krelborn demands he take care of the plant because it's the only reason his little shop is still in business. Reluctantly, Seymour knocks off some more people and feeds them to the growing plant.

Soon, Audrey II is enormous and Seymour is afraid there's no stopping the plant. He confides in Audrey the secret of the plant and offers his undying friendship; she is touched by how well he treats her - no one ever has before. They make a plan to run off together, but first Seymour has to take care of Audrey II. The problem is that Audrey II has grown to enormous size and has already started budding, producing additional little flytraps who need blood too. Soon, she'll be big enough to move beyond skid row and in the city, taking it over, and eventually, the world! (She is a mean, green mutha from outer space, after all...) Seymour won't let that happen though, and after a brutal fight that tears down the Musknik flower shop, Seymour electrocutes Audrey II, blowing her into a million pieces. Seymour and Audrey go off to live in a little house with a white picket fence, and for Audrey II, suppertime is cancelled... permanently.