Crystal, Ronette and Chiffon, a street-wise, sixties-style girl group, narrate in song and escort us on a journey into an American city, where life is tough for the inhabitants of Skid Row.
Mushnik's florist, whose basement is home to the nerdy, orphaned Seymour Krelborn, is on the verge of going bust. Then Seymour brings out a strange and interesting plant that he's been cultivating, which he bought from an old Chinaman during a total eclipse of the sun. Almost instantly, the curious plant begins to attract customers to the shop, thus saving the business. Seymour christens the plant Audrey II, after his heart-throb, Mushnik's faux-glamorous shop-assistant. However, frequent bouts of wilt look set to jeopardise their success, until Seymour finds, when he pricks his finger on a rose thorn, that the plant needs to be fed with blood.
Meanwhile, Seymour's love for Audrey goes unrequited in favour of her abusive boyfriend Orin, a sadistic dentist.
As the plant's notoriety grows, business booms, and Seymour becomes a celebrity appearing on talk shows, and featured in Life magazine. However, as it gets bigger, so do it's demands for blood. Magically the plant begins to talk, its pleas to "Feed Me" exceeding what Seymour alone can provide. So, when Orin meets an untimely end, the plant gets fed, and Audrey's attentions are free to turn to Seymour.
As the plant's thirst escalates, Seymour finds himself resorting to ever more desperate measures to find food for it. As those around him, gradually, become sucked in (literally) by the plant's appetites, Seymour has to choose between having fame (at a price) or making the ultimate sacrifice.