Set in the far future, an evil god known as the bios has destroyed much of the earth, turning it into a desolate wasteland known as the world of dust. Two unnamed super soldiers are created by humans to defeat the BIOS and the eight evil gods that serve him.
Forgotten Worlds can be played by up to two players at the same time. The player controls a flying muscle bound soldier armed with a rifle with unlimited ammo. Player 1's character is equipped with a long-range automatic rifle, while player 2 has a short-range wide shot. The controls in the original coin-op version consist of an eight-way joystick for moving the character in the air while flying, and a unique rotary button known as the "roll switch". Rotating the switch left or right allows the player to set their character's aim in one of sixteen directions, while pressing it causes the character to fire their pistol. This allows the player to move their character anywhere while keeping their aim in the same direction.
The player character is accompanied by a satellite pod orbiting them that will provide backup firepower every time the player fires their gun. Like the main character, the satellite can also be rotated using the roll switch. Rotating the character while firing will only rotate the target of the satellite, while rotating the character without firing will not only rotate the target of the satellite, but also move its relative position around the player.
The player can obtain blue coins known as Zenny from defeating enemies throughout the game. Zenny is used as a currency to obtain new power items from shops located at certain points in each stage. When the player enters the item shop, they are given a choice of available items and a limited amount of time to make any purchase they want. These items mainly consist of new weapons for the satellite module, but also include a first aid kit to restore lost vitality, a weapon that allows the player to take extra damage, and even tips on how to defeat the waiting boss at the end of the current stage.
Forgotten Worlds consists of nine stages, each with its own boss. The player will lose if their vitality runs out, but will be given a chance to continue.