The day Lion Biyela graduated from the Naval Academy as a Marine officer, he resigned his commission and enlisted in the Interstellar Marines as a grunt. He has never regretted it.

Several years later, Lion is now a sergeant, leading his own fire team. The Marines of his team are his friends and family, his brothers and sisters. They serve on the colonial world of Eos, where they fight against alien invaders seeking to destroy all things human.

But it's not just the war and the invaders that they strive against, but rather cold-hearted fellow Marines, self-righteous officers, and the fear of losing each other.