Galaga ’91s plot involves an intergalactic organization, the United Galaxy Space Force, deploying a lone starfighter to rid the galaxy of the hostile Galaga aliens, who plan to eradicate all of mankind.
The player controls this starfighter throughout eleven different stages, each becoming progressively difficult. Stages are indicated by small emblems that are located at the bottom-right of the screen.
The final stage has the player facing off against a large boss enemy.
In each level, the objective is to destroy all of the Galaga aliens, which fly into formation from the top and sides of the screen.
As in other Galaxian games, enemies will divebomb towards the player and attempt to collide with their ship while also firing projectiles. Atop the formation of enemies are four larger aliens known as the Boss Galaga, which take two shots to destroy.
During a divebomb, they will sometimes use a tractor beam that can capture a player’s ship and return with it to the top of the formation, costing the player a life.
The player is able to shoot down a Boss Galaga holding the captured ship to rescue it and transform the player into a “dual fighter” with additional firepower and a larger hitbox.
The player has to shoot the Boss Galaga holding the captured ship while it is divebombing; shooting the Boss Galaga while it is in formation will instead cause the fighter to turn against the player and act as an alien.
The third, eighth, and twelfth stages are bonus levels, which are indicated by the text “That Is Galactic Dancing”.In these, enemies fly into the screen along pre-set paths without firing any projectiles.
Players can shoot down the aliens to earn bonus points; destroying all enemies awards the player a large sum of points. Some stages are vertical-scrolling corridors, where players must avoid constantly-moving waves of enemies and other obstacle