An abstract one or two player shoot-em-up in which the aim is to stop swarms of aliens from building a sectional tunnel from the bottom of the screen to the top.

At the top of each level are blue tunnel sections, while at the bottom are purple ones. Aliens start at the bottom of the screen in the purple sections. As they move through a tunnel, they can build new sections in any direction.

Players can either destroy blue tunnel sections, or 'shove' them to re-align them and prevent the aliens from completing their mission. Purple sections cannot be destroyed but they CAN be shoved. A 'smart bomb' can be deployed which will destroy all nearby enemies, as well as the players, on-screen counter-parts should they be within the blast radius when the bomb explodes. Additional bombs can be earned by destroying blue sections of tunnel.

If the aliens succeed in connecting up a tunnel, additional enemies appear and the challenge increasing substantially. Each round ends when a preset number of 'Blocker' aliens have been destroyed.

There are four types of enemies :

* BLOCKERS - These foes build the tunnels and they can exit the tunnel system or be forced out. If they exit the tunnels, they produce 'Expelers' (see below) and quickly make their way to the bottom of the screen to re-enter the tunnels. This is the only time that they are vulnerable.

* EXPELERS - These enemies, produced by the Blockers, chase the players around the screen and attempt to corner them. They travel at different speeds, generally moving faster as they get closer to SWAT agents.

* ANNGRERS - These cubed-shaped foes are invulnerable to the SWAT agent's fire. Shooting this type of enemy will cause it to chase the players very quickly. They can be destroyed by using bombs, but a new Anngrer will be created in its place at the bottom of the screen. These enemies can travel in and out of the tunnel system wherever they choose.

* SWEEPERS - Along with additional enemy fire, these foes appear in pairs on the left and right sides of the tunnel along if the bottom and top portions of the screen are connected by one continuous tunnel. They will line up with each other on either side of the players' SWAT agents and fire at them. Sweepers require multiple shots to be destroyed.