80's classic arcade game.
The player begins the game with a small spaceship armed with a single laser cannon. After successfully completing the first four waves of alien attacks, the player must attempt to dock the ship with the next 'stage' of the ship. This second stage has two lasers in addition to the original one. Each docked stage is one of the player's "lives."
After successfully clearing two more waves of aliens, the player must again dock with the third and final piece of the ship, which also has two more lasers (giving the player five lasers in total). The trade-off for this is that the entire ship is a much larger target. Failure to correctly align the stages during either docking sequence causes the destruction of the stage being docked with.
After completing the first eight waves, the player's ship reverts to the first stage and the process is repeated. If any of the player's three ships are lost along the way, the docking sequence occurs only after the first four waves have been completed. Play ends when all three of the player's ship-stages are destroyed.