The game opens with the player above Earth with the message "Defend the Earth from comets!". The player ship must navigate two stages per planet in the solar system As enemy waves spawn at the player, all surviving enemy ships, and in rare cases, shots, will proceed past the player down a mini-map on the right-hand side of the screen and impact the planet. Each impact causes a 1% increase to the planet's damage. If the planet hits 100%, the game is over, regardless of the remaining stock of player lives.The first stage of each planet is divided by a mini-boss fight. This encounter is a large ship that fires large bullets in clusters at the player while standard enemy waves continue to spawn. The second half of the first stage ends with an encounter with the "comet" which will share the same colour as the planet being defended (blue for Earth, yellow for Venus, Red for Mars, and so on)