Alpha Mission (also known as ACO II: The Last Guardian, its Japanese name) is a vertically scrolling full-screen shoot 'em up published by SNK Corporation in 1991 for the Neo Geo arcade and home systems. It was later released for Neo Geo on CD in 1994 and on a handheld game console in 2010.
The player controls a spacecraft fighter (tank object skirmish) and can shoot enemies in the air, bomb enemies on the ground, collect bonuses and defeat bosses to advance levels.
The game control uses 2 or 3 buttons, used depending on the selected transfer mode. The weapons of the ships consist of air-to-air laser shots, air-to-ground missiles and any power weapon the player collects. Laser and missiles can be upgraded four times by collecting bonuses from Drones that are removed. Weapon power is available after players collects three tokens of the same type of armor or can be purchased with credits acquired after each stage. Weapons have limited power, indicated on the energy meter. Using a weapon or a damaged ship lowers energy. Energy can be replenished with E bonuses. As soon as the energy expires, the ship returns to normal mode. Depending on the selected mode, it changes whether the C or B button activates the weapon's powers.
Various hidden elements can do certain things, like the R element can put the bus in the opposite direction for a certain distance. The shuttle is destroyed with one shot or run into the enemy or danger. If the player collects enough powerups to advance, missiles and laser shots will be lost to the minimum power when the player loses a life.
There are seven stages in the game, with the last one consisting of a single battle. The player must fight through them while avoiding dangers in addition. Bosses very rarely appear in the game. When the player encounters a boss or end stage boss, the player must destroy weak points and various parts of the boss in order to destroy it.