Seymour Krelborn, a clumsy florist shop assistant, finds an unknown plant that looks like "a cross between a Venus flytrap and an avocado". He names it Audrey II after his coworker, Audrey, whom he is in love with.
Seymour soon finds out that Audrey II requires blood to live. He feeds it, and it grows to monstrous size. Audrey II reveals that it can move, talk, and sing (in a very deep, commanding voice) and demands more blood. This leads to Seymour having to feed people to the plant. He does this willingly because the plant has brought him fortune and fame, love and money and instant acclaim.
But, despite all that Audrey II offered Seymour, the moral of this musical is: "Don't Feed The Plants!"