You're a cowboy fighting off the Slymoids with your gun. They're coming in from the horizon and from the sky. When coming in from they sky they move about from side to side as they slowly descend to the bottom of the play field. You can shoot up in the air at the flying Slymoids, or left and right at the ground dwelling Slymoids once they've reached the front of the screen where you walk to the left or right on the ground.
As you play, the game follows you to the left and right, scrolling through each of the three locations you can play in: along the coast line, in the city, and in the cavern. The cavern has an extra bit as they add strange live dropping stalactites that follow you once they've landed on the ground. Don't get too caught up shooting, you have to stop shooting to move and the Slymoids are coming from all directions but in front of you. You can start the game at any difficulty between 1 and 4, and have three lives.