"Dark Horizon" is a continuation of the Tarr Chronicles series, known as "Tarr Chronicles: Sign of Pride". Although the term "sequel" indicates a plot connection, the action here takes place almost 100 years earlier than the events presented in the previous part, "Sign of Ghost".

The action of the game begins when the fleet of the dangerous De'Khete race appears in the areas of the galaxy controlled by humanity. In a short time, hundreds of cities and bases are destroyed, and humanity faces a second era, called the De'Khete Age. The player takes on the role of a defender of humanity, whose task is to face this deadly race and prevent them from building a device that spreads Darkness within several star systems.

"Dark Horizon" introduced a new race called Sentinels. They were once humans corrupted by the Darkness, but they managed to find the strength to oppose De'Khete, creating their own culture, world, and technology. Their advanced spaceships, with powerful armor and weapons, are perfect for fighting the invaders.

One of the unique gameplay elements are the Stealth and Berserk modes. Stealth mode allows ships to become invisible to the enemy. Berserk mode, on the other hand, increases firepower, allowing the player to concentrate a powerful attack on the enemy. The efficient use of these modes becomes the key to success in the difficult battle against De'Khete.