- The narrator (Stanley Jones) tells of an event that once threatened the existence of the entire human race. It occurred in "the most seemingly innocent and unlikely of places", Skid Row, in New York. Three teenage girls (Michelle Weeks, Tisha Campbell-Martin, and Tichina Arnold) appear throughout the film as a Motown-singing Greek chorus. They set the scene at the unassuming Mushnick's Flower Shop ("Little Shop of Horrors").
Seymour Krelborn (Rick Moranis), the meek, clumsy employee of Mr. Mushnick (Vincent Gardenia), lives a dreary life among the other unfortunate residents of Skid Row ("Skid Row/Downtown"). Taken in by Mushnick as a young orphan, Seymour lives in a tiny room below the flower shop and is treated fairly poorly by his employer. The light of his life is his co-worker, Audrey (Ellen Greene), a fragile, battered woman stuck in an abusive relationship. The flower shop is making virtually no money, and Mr. Mushnick is on the verge of closing it for good when Audrey mentions the strange and interesting new plant Seymour has been developing, and suggests that if it is properly advertised, it might attract more business. Seymour presents the plant, a large bud surrounded by layers of leaves. Mushnick is pleasantly surprised when customers begin flocking in after seeing the little plant in the window. Seymour has no idea what kind of plant it is, but explains how he got it ("Da Doo"): While he was shopping for odd new plants, his hobby, a solar eclipse occurred. After daylight returned, the strange plant seemingly appeared out of nowhere. Seymour bought it for cheap and brought it home to examine. He shyly admits that he named it "Audrey II."
The shop is doing record business until the plant begins to wither. Seymour frantically tries to nurse it back to health, though water, sunlight, fertilizer, and minerals have done no good ("Grow For Me"). When he pricks his finger on a rose thorn and draws blood, Audrey II responds vigorously. Seymour realizes what it lives on, and reluctantly gives it a few drops. Audrey II has a dramatic growth spurt and business picks up, with Seymour becoming a local celebrity ("Some Fun Now"). He does not reveal the secret behind the plant's health.
Audrey's boyfriend, Orin Scrivello (Steve Martin), is a sadistic dentist with a leather jacket, motorcycle, and ducktail haircut. He adores his profession because of the constant opportunities to inflict pain on others ("Dentist!"). He enhances his pleasure through his habit of huffing nitrous oxide, or "giggle gas", for a euphoric high. Seymour and Mushnick are concerned, as Audrey often comes to work with injuries or a black eye. Orin arrives at the shop one night to pick up Audrey, and Seymour sees first hand how roughly he treats her. Audrey, meanwhile, secretly dreams of an idyllic suburban life with Seymour ("Somewhere That's Green"), but is too frightened to leave Orin.
Seymour angrily paces around the empty shop at night, venting his frustrations to Audrey II, who is now roughly the size of an armchair. To Seymour's great surprise, Audrey II responds with a human voice (Levi Stubbs) and demands more food. Knowing that the plant can no longer live on mere droplets of blood (partially because it is too big, and partially because Seymour is now weak and anemic from feeding the plant from his bleeding fingers), Seymour is reluctant to provide it with bigger portions. The smooth-talking Audrey II promises Seymour money, fame, and the girl of his dreams if he continues to nourish it with human blood ("Feed Me (Git It)"). Seymour initially refuses to murder for the benefit of Audrey II, reasoning that he doesn't know anyone "who deserves to get chopped up and fed to a hungry plant." Audrey II directs him to the window, where he sees Orin assaulting Audrey across the street. Filled with rage, Seymour agrees to bring back a specific meal for the plant.
The next day, Seymour goes to Orin's office with a gun in his coat pocket. The patient before him, Arthur Denton (Bill Murray) is a gleeful masochist who frustrates Orin by actually enjoying the oral torture he receives. After Denton's departure, Orin drags Seymour into his office to get the sadistic fix he had been craving. Though he wants to spare Audrey further pain and has promised Audrey II fresh blood, Seymour is terrified of shooting Orin. Conveniently, Orin dons his special gas mask, which provides him with constant nitrous oxide and increases his sadistic pleasure. The mask malfunctions and Seymour watches his nemesis asphyxiate before his eyes. Seymour drags the body back to the flower shop, where he reluctantly chops it up with an axe and feeds it piece by piece to Audrey II.
Seymour is understandably shaken by the incident, and sees the police speaking with Audrey the next morning about Orin's disappearance. He tries to comfort Audrey, telling her that she had deserved a better man all along. While Audrey admits that her low self-esteem kept her tied to Orin, she feels partially responsible for his fate because she had been silently wishing him out of her life. Though he does not reveal the truth about what became of Orin, Seymour promises to stand by Audrey no matter what ("Suddenly Seymour").
Invigorated from finally winning Audrey's love, Seymour returns to the flower shop. He is confronted by Mushnick, who had seen Seymour dismembering Orin's body the night before. Mushnick assumes that Seymour's devotion to Audrey caused him to murder the cruel dentist, but does not suspect that Audrey II is involved, and knows nothing of its grisly diet. Mushnick leads Seymour upstairs at gunpoint, planning to turn him in to the police. He then offers Seymour a proposition: Seymour leaves town forever, and Mushnick keeps the money-making plant. As Audrey II coaxes Seymour to sacrifice Mushnick to clear his own name ("Suppertime"), Seymour nervously backs Mushnick into the gaping maw of the hungry plant, killing two birds with one stone.
With Audrey II grown to enormous proportions, Seymour's celebrity increases ("The Meek Shall Inherit"). However, he is miserable and terrified of the plant's eating habits getting out of hand. He begins to fend off the media and turn down large sums of money, feeling guilty for his part in the secret deaths of Audrey II's meals. He finally puts his foot down, denying the plant any further feedings. He and Audrey plan to run away that night and get married, escaping Skid Row forever. However, he buckles when Audrey II seemingly agrees to accept some meat from the butcher instead of a human corpse. Seymour falls for the ruse and leaves the shop. The plant, with its fully mobile tendrils, uses the phone to call Audrey's apartment across the street, begging for water. Audrey, in her wedding dress, comes to oblige, and is amazed that the huge plant can talk. As she fetches the watering can, the plant wraps its vines around her legs and drags her into its mouth ("Suppertime II").
Seymour returns just in time to save Audrey from being devoured. Outside the shop, he explains how dangerous the plant is, and how he only kept it because he thought Audrey would cease to like him if he were no longer rich and famous. Audrey assures Seymour that she loved him from the moment she met him, and still does. They are interrupted by a salesman (James Belushi) who shares with Seymour his idea of collecting trimmings from the plant and marketing miniature Audrey IIs all over the country. Seymour deducts that this was the plant's plan all along: It was an alien that came to earth during the solar eclipse and was bent on world domination. He rushes back into the shop for a final showdown with Audrey II. The plant thwarts Seymour's efforts at killing it, and begins to taunt him as its branches destroy the building and new buds begin to bloom ("Mean Green Mother from Outer Space"). Emerging from the rubble, Seymour grabs a live electric wire and touches it to Audrey II's vine, electrocuting it and causing it to explode into nothing.
Seymour and Audrey are married and arrive at their suburban dream home, just as Audrey had imagined. As they enter the front door, a baby Audrey II plant is seen in their garden, a tiny smile on its pod