Mercenary Force is an unusual shoot'em up set in Japan's Edo period and inspired by Japanese history and mythology. When Japan is invaded by the minions of a dark lord, the shogun sets out to hire the best warriors in the land to combat the enemy.
The mercenaries must shoot hordes of enemies to get to the next level, including several bosses. They can change their formation between four different options: a vertical line, a diamond shape, a square shape and a horizontal arrowhead formation. It is also possible to switch the character in the lead position, leading to a large variety of tactical options.