From the back of the box:
You emerge from beneath the sea to save the planet from the Global Armed Alliance, a ruthless clan of terrorists who are prepared to take over the world. The submarine you command is the fiercest ever known to naval warfare. Your mission is to seek and destroy the enemy forces. You chart your course around the world to regain major ports that are under terrorist control.
TORPEDO RANGE is a two and three dimensional underwater action adventure for your Game Boy® unit. As you defeat each enemy, you will be able to upgrade your sub with more firepower and additional shields. TORPEDO RANGE continuously monitors your performance and will adjust according to your ability.
So full speed ahead, up periscope and get into TORPEDO RANGE.