Based on the game Space Hulk, it allows you to play as alien assault team taking on a derelict space ship.
After the contamination of the Genestealers' cryogenics chambers, the remaining Genestealers launched a last wave attack against the Blood Angels' defensive perimeter. Several of the Blood Angels' Command Units took to the field to bolster the perimeter, beating off the attack with some difficulty. The Blood Angels' force commander feared that further Marine losses might force them to abandon the hulk altogether.