Side-view guy-with-gun action similar to all those side-view Nintendo games. Lots of different weapons and items, levels scroll in all directions. Or to copy from the better summary that I wrote for the front page: “gameplay like Rolling Thunder, an open world like Blaster Master, an assortment of weapons and items like Metal Gear, a leveling system like Zelda 2, and a really silly plotline.”
Earth is being invaded by a race of idiots who find the Earthlings to be an even match. After an enemy dreadnought blows a chunk out of the Moon by powering up to fire on the Earth and forgetting to get out of the orbiting Moon's way, Earth's generals decide to call up the retired hero of World War 3, Colonel Bob Smith, to attack the aliens' home planet. The post office delivers the letter to a random guy named Bob Smith. The generals decide to send him anyway.
Bob can jump, duck, run, walk, punch, and dive. He can find items to extend his health or lungpower (the need for lungpower is stolen from the old TRS80 game Volcano), restore his health when injured, give him a boost of speed or strength, etc. Bob has meters for life, energy, and air. Energy is expanded through running, physical stress, and taking hits that aren't powerful enough to deduct life. Unlike NES games, bumping into an enemy doesn't mean loss of life, just that Bob and the enemy bounce off each other and land on their asses.
There's a lot of humour in the game; One of the bosses is a giant smiley face, a later boss is the same smiley face with buzzsaw blades for an edge. Bob and the aliens can get folded, spindled, and mutilated in cartoony ways.
There are also secrets: A boss tank whose windshield can be punched through to drag the driver out and win instantly, for instance. (the driver ducks under other fire)
There'd be a sort of game map where certain levels connect to others in specific places, similar to Blaster Master or other games where levels aren't straight in order.
A possible “plotline” for ordering the level-to-level action involves the alien planet being so horribly polluted that Bob literally cannot wander from certain areas to others, but there are giant disabled filtering machines that he can repair and turn on.
The game's last level involves Bob sitting on the Big Planet Destroying Missile that's flying towards Earth. He has to reprogram it while watching for and ducking under asteroids that'll knock him off. There are different endings depending on where it gets reprogrammed to go.