Before a simulation war between two teams of recruits began, a group of alien force passed the sentinels and snuck onboard a Cluster Singlestage Boat heading to Mars. The alien destroyed the ship, making it crash on the surface before they came out from the wreckage to raid the Martian Base, killing all the personnel in the base.
After Sergeant Zim receive Juan Rico's status report, he called for the backup for several times. However, the command asked for more proof. Zim had no alternative but to directly called the Mobile Infantry, asking them to descend on Mars.
Later, the alien force went outside the base and attacked Mobile Infantry trainees, killing several of them. The Mobile Infantry arrived afterward and kill all the alien invaders.
Greg Paterson, a fellow trainee of Juan Rico, was among the dead trainees as he was killed when trying to save Rico.