The Delphins share a planet with the Halons. Your side of the planet, the one inhabited by the Delphins, is constantly engulfed in total darkness but the Delphins have adapted to this life. The Halons used to live in the light but recent events have caused their side of the planet to fall dark and now they are attacking you in an attempt to take over your technology. As a Delphin pilot, your job is to stop them and take down of their attacking ships which is what most of the gameplay consists of; finding enemy ships and killing them.

Darker's story is revealed over the course of the game as information gathered by reconnaissance. The people of Delphi are not militarised, having never had reason for military action, and so technology for combating the Halon menace is developed over the course of the game. The Halon people are revealed to be invading in order to expand their cities away from the light side, which they have polluted beyond the point where it can sustain life. Eventually Tolly pilots a Halon craft to the light side of the planet, where he destroys the Halon aircraft factories.