Seymour finds a seed from 200 million years ago and helps it grow into the plant known as Junior. The plant tries to return home but, upon finding that he cannot, stays with Seymour and helps Mr. Mushnik's flowers grow.

Seymour joins a home economics class where he and Junior conspire to make a human–plant hybrid a la Frankenstein. Meanwhile, Audrey decides she wants to be a refrigerator repairman.

Seymour's science project, which was made by Junior, wins a science fair and attention from Seymour's favorite celebrity. However, the scientist wants to steal Seymour's project and use it for himself.