The action opens on Mushnik’s Flower Shop, a failing store on Skid Row. The few plants in stock are, like their surroundings, run down and fading. There we meet Mr. Mushnik and his two employees, Audrey and the nerdish Seymour Krelborn who secretly has a crush on his beautiful but much-abused co-worker.
Audrey, attempting to help Mushnik bring more customers into his failing business, persuades him to look at one of the “exotic plants Seymour has been tinkering around with.” Mushnik is intrigued and allows Seymour to display his flytrap-looking plant, which he has named, in honor of his secret love, Audrey II.
Seymour and Crystal, Ronnette, and Chiffon (who provide musical accompaniment and commentary throughout the show) perform an upbeat number explaining that the strange flower was purchased from Chang’s Flower Shop. The plant appeared, “mysteriously,” after a total eclipse of the sun.
Of course the strange plant draws customers and business is soon booming. In celebration Mushnik invites Seymour and Audrey out on the town, but Audrey has a date with her boyfriend, the sadistic dentist, Orin Scrivello. Seymour also declines and stays behind to tend to the suddenly ailing Audrey II. That night, alone in the shop, he discovers the shocking secret to the plant’s health and phenomenal growth: human blood.
Seymour encourages the plant’s growth by pricking his fingertips and feeding Audrey II his own blood. This only lasts for a few days, however, and the meager drops of blood aren’t enough for the quickly growing plant. Finally, late one night the plant grows bold and speaks: “feed me.” Audrey II has now become a singing, dancing, jive talking, vampire vegetable!
Eventually, the plant demands more and more, and Orin becomes more and more abusive to Audrey; thus Seymour comes up with the natural solution. Murder Orin and feed him to Audrey II! He is spared the ghastly deed, however, when the nitrous-addicted dentist laughs himself to death on his own anesthetic gas. Seymour is left only with the task of dismembering the dentist and feeding him to Audrey II.
This, of course, also opens up interesting possibilities in Seymour’s love life. Following the disappearance of Orin, Seymour builds up the courage to tell Audrey of his true feelings for her. Audrey responds with the touching song, “Suddenly Seymour,” and the two are united.
Unfortunately, Mushnik witnessed Seymour’s crime and threatens to turn him into the police if he does not leave the country. The plant, however, must stay. Of course the solution for Seymour is once again right in front of him, and he tricks Mushnik into the jaws of the ever-hungry Audrey II. As the plant and the body count grow so does Seymour’s fame and fortune.
Ultimately, however, Seymour’s natural goodness begins to make him think about his lifestyle and what he must do to maintain it. At last, he decides to flee with Audrey to be married and plots to destroy Audrey II. Before Seymour can complete his plan, however, the plant mortally wounds Audrey. Dying, she requests that she be fed to the plant, so she can become a part of it and always be near Seymour. As the music swells, Seymour feeds her to Audrey II, which at last reveals its ultimate plans--nothing less than world domination. Seymour makes one last attempt to kill the plant but fails. In the end, he too is devoured. The singers, joined now by the faces of the dead characters, warn that Audrey II and other alien plants have begun to devour the world one city at a time--and we are left to scream with laughter!