Just like in the arcade version, the object is to drop depth charges on fleets of enemy submarines as they glide beneath your battleship at several different depths. There's a radar screen at the bottom of the playfield that picks up subs outside of the battle area, allowing you to anticipate their arrival. There are also bonus subs that cruise past along the bottom; they're very hard to hit and are worth more points the longer you wait before nailing them. The main challenge in Sub Scan is in the way it forces you to make judicious use of a limited supply of depth charges that drop very slowly through the water. You only get four at a time, and the ones you've fired have to explode before your supply is refilled. This feature puts a high premium on making every shot count.
Just about everything, from the graphics to the sound, are translated nicely on the 2600 translation. What's wrong? Simple: In the coin-op, the subs fired torpedos back at you. In the cart, they don't. This problem is magnified when you realize what a simple game the arcade version already was to begin with. Because of this glaring omission, Sub Scan becomes nothing more than target practice. The home version presents you with just the one challenge -- and a limit of ten subs. Once they all get safely across the screen, your ship simply blows up and the game's over. After just a few plays, Sub Scan becomes as challenging -- and as pointless -- as shooting plastic ducks with an air gun.