In the introduction animation, a young girl receives a gimmick doll named Yumetarō (ゆめたろー?) for a birthday present. She quickly favors the new doll over her previous toys who, jealous over abandonment, kidnap her and hold her captive in an alternate dimension inhabited only by toys. Yumetarō, as the protagonist, must track down his new owner and retrieve her from the toy world.
The player controls Yumetarō, who uses a star that spawns above his head to defeat enemies found in the game. The star doubles as a functional utility, as Yumetarō can ride his creation and use it to reach otherwise unattainable areas.
Yumetarō must make his way through six stages and six bosses to complete the game. Once the player completes the six stages, it is revealed that the girl is still missing. To fully complete the game, Yumetarō must go back and find a hidden area in each stage where a magic item resides. If he can obtain each stage's magic item without losing all his lives (i.e. no continues allowed), a secret stage will be made available where an extra boss must be beaten. Only after these conditions are met is the game completed in full, with a new animation showing Yumetarō rescuing the girl and leading her back to safety.
Source: Wikipedia, "Gimmick!", available under the CC-BY-SA License.