Neptune is a side scrolling arcade game where the player controls a submarine while destroying or avoiding underwater sea creatures.

The submarine can move in all 4 cardinal directions, while the ground and monsters rapidly scroll by from the right. The submarine has an unlimited number of laser shots which shoot out horizontally from the vessel, as well as a limited number of bombs, which descend vertically. The are multiple levels, and each level has several stages, including underground components and differing enemies. The player receives 20 bombs per level. The player has limited fuel, but will refuel at the end of each level. Enemy creatures consist of various underwater creatures, including sharks, jellyfish, and more. If the player hits a creature or the ground, they lose a life. The player starts with 5 lives, and it's game over when all lives are lost.