The Promise takes place in a futuristic Earth already well-versed in space exploration and contact with alien species. One of these species, the Klix, waged war on the human race a long time ago, and the only way Earth’s forces turned the tide of the battle was by genetically engineering a group of fast-aging soldiers, the Sixers, who fought like savages. In the wake of these specialized battalions came the Piecers, a group of genetically modified troops who aged at the normal rate. Most of the Piecers never saw battle since the Sixers drove the Klix off of Earth. But on the day the Piecer Corp is to be disbanded, the Klix bomb the program’s grounds, including residential neighborhoods.

A small team of Piecer soldiers—Chase, Tripp, Gage, and Hector—search for their families in the wake of disaster while trying to apprehend the last few starships so they have a fighting chance against their returned enemy. Chase travels about the solar system, stopping at bars on a moon and fighting with hostile aliens as he seeks to repair his ship’s engine and attain more fuel. After stealing back The Lone Star, the team all rendezvous and plan their attack on the Klix, preparing to risk their lives to put an end to the Klix tyranny. First, they must uncover the traitor and prove their leader’s motto: "No crewmember left behind."