Galaxy Invasion is a Galaxian-style arcade game for the TRS-80. The basic gameplay is the same as the arcade game, with the player's spaceship at the bottom of the screen facing off against waves of diving alien ships.
However, a twist is added to the game with the inclusion of Big Five's signature Flagship alien. At the start of the game there are two Flagships with escorts along the top row of the screen, as in the arcade game. However, as the player advances through the waves of alien ships, more and more Flagships are added to the top row until it is filled with Flagships.
A "Flagship Attack Alert" is displayed once too much time has passed without a Flagship being destroyed. If the alert ends before the player can destroy a Flagship, then all of the Flagships on screen fire their laser bolts, automatically destroying the player's ship.
Galaxy Invasion was the first Big Five game to feature sound effects. These were generated through the TRS-80 cassette output port and could be heard by plugging the cable into an amplified speaker.