Circus Caper is the title of a side-scrolling NES game where the player controls a young boy on a quest to save his sister who has been kidnapped by the circus. The game was released in 1990 by Toho Company Ltd. and received poor reviews.
The single-player, side-scrolling action game takes the player through various levels, each with a circus theme, fighting various enemies and bosses until he finally defeats the ringmaster and save his sister.
The player start out in the game with little life, and few weapons, but he can collect various icons to increase how many direct hits that he can withstand, keys to open doors, along with various circus items to shot as projectives.
Many of the levels can be avoided altogether by just walking backwards and back through the curtain from where the player begins. This will take him right to that level's boss battle.
Bonus levels include such fun things as guiding a bear to jump over flames, avoiding boulders in a car, using Rodan as target practice and meeting Godzilla at the circus grounds.
Source: Wikipedia, "Circus Caper", available under the CC-BY-SA License.
It's worth noting that the game developer was the company responsible for the Godzilla films, this makes the last paragraph of the story description much less bizarre.