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 (including Pluto, as the game was released well before the re-classification of Pluto in 2006.) 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.