The story of the Lost Marines is a sad one. The real name of the Chapter has since been lost. They were one of the original 5 chapters. A strong and loyal race of men, until the Horus Heresy that is. Days before the rebellion began, half of the Chapter's Marines, Terminators and Scouts had been sent to cleanse a planet of an extremly large force of Orkish invaders. While they were there a group of Horus' followers landed on their homeworld that now doesn't exist. Some of the chapter members left on the planet were turned to the forces of Chaos. These troops then led a succesfull coup against the remnants of the Chapter.

The force that was sent to fight the Orks returned to find their world to be the rebels' HQ. Their Primarch and his few loyal followers were dead, and the Great Abbey destroyed. The still Imperial Marines left for the Emperor's palace to report these occurences. The Emperor allowed them to remain in orbit but didn't think this revolt would amount to much. He sent the Alpha Legion to deal with the insurrectionists. The Legion disappeared soon after. Other Chapters went missing as well.

Then impossible happened, a fleet of transports and warships jumped out of warp just outside the Home System. This fleet destroyed almost all the ships in orbit. The Lost Marines ships were able to jump out before being destroyed. Before leaving, however, the ships dropped all the units on board to the surface. These units became instrumental in the war for control of the Imperium. At the battle for the Emperor's gate the Lost Marines, with the help of Home World Imperial Guard units, held the forces of Chaos for three hours after the other Chapters had fled behind the walls of the Palace. The half-Chapter stood in the face of heavy losses, only 50 of the 500 Marines survived along with a small amounts of Terminators, Scouts, and Imperial Guardsmen, until the Blood Angels could lead a counter-attack. Before the battle was won, the heroes of Emperor's Gate had disappeared. They were presumed dead, but a few knew the truth....

Since then the Marines have grown back to a force of 500 Marines. They refuse to go back to full size until they have eradicated all of the Imperium's enemies. This does not include Scouts which they have lots of, but does include Terminators. They have also created their own Imperial Guard force upon the remnants of the Home Guard. They fly through the Galaxy and fight any force that threatens the realm of the Emperor. The Lost Marines do occasionally fight alongside any native troops or other Chapters but leave no evidence it was them. As the Imperium continues it's centuries long wars, the Marines fight enemies both in the Imperium and the fear they may one day fight their Chaos brothers. So the Chapter stands as an invisible wall for the Emperor's people. How many times has the Chapter saved the Imperium from doom? It is hidden deep within the records of the Emperor's lost children.