- "Today we're gonna paya visit to Bug City.We're lookin' for a bug o one's ever seen before.Some kind of smart bug.And it probably knows we're comin'"
- —Johnny Rico
After the Ambush at Whiskey Outpost, Military Intelligence had confirmed that there was a Brain Bug on Planet P. Therefore, they had an invasion begun that involved the Mobile Infantry to go to Planet P and capture the Bug.
During the initial stages of the battle, Bug plasma hit the Rodger Young, tearing it in two. Carmen Ibanez managed to flee the ship in an escape pod with Zander Barcalow, while Captain Deladier was killed by a closing door. However, they were then forced to crash-land on the planet's surface. Upon trying to land, they crashed into a Bug lair where they are then captured by Bugs.
Meanwhile, the Roughnecks, having just received new recruits straight out of training, heard not only Carmen Ibanez's distress call, but also them getting attacked by Bugs before losing radio contact. Roughnecks leader John Rico initially decide that they must already be dead and continues on the mission. However, he subsequently changes his mind and orders two volunteers to follow him down a path, while the rest of the Roughnecks were told to proceed to their main objective.
Meanwhile in the cavern where they were captured, from behind the Warrior Bugs appears the Brain Bug. It moved over to the two crew members and proceeded to suck out Zander Barcalow's brain with a large proboscis from the center of its head.
The Roughnecks were moving down a cavern when Rico stopped at the entrance to another tunnel. He asked for two troopers to go with him down the second tunnel and orders the Roughnecks to continue on the mission.
The Brain Bug, after having killed Zander Barcalow, prepares to kill Carmen Ibanez for her brains, but she managed to cut the proboscis off the Brain Bug with a knife. The Bugs were ready to kill Carmen Ibanez when the Brain Bug ordered them to stop very suddenly. Rico, along with Ace Levy and Sugar Watkins, appeared at an entrance to the cavern with a Tactical Oxygen Nuke in his hand. Carmen was rescued and grabbed a Morita Assault Rifle, staying with Rico. The Brain Bug then recognised the threat and retreated out of the cavern; then once safely out of the way, the Warrior Bugs attack forcing Carmen and the Roughnecks to withdraw. Mortally wounded with a slash across his chest, Watkins took the nuke whilst holding of the Bugs long enough for the rest of the party to flee far away enough before it went off.
Upon returning to the surface, Rico, Carmen, and Ace found many troopers celebrating and running to a tunnel entrance where the Brain Bug was captured in a large net and being dragged out. Carl Jenkins appeared with several military personnel and connected himself to the Brain Bug; through this, he was able to sense that the Bug was scared, which delights everyone around them.
Jenkins then walked off and meets Rico and Carmen. Rico said that he believed that it was Carl who showed him where to go and rescue Carmen, which Jenkins just responds that it's classified.
Jenkins then pointed out that the trooper who captured the Brain Bug turned out to be Zim, the very man who had trained Rico before the war but now a private in a combat position of the Mobile Infantry. Rico Congratulated him.
It was believed that the Invasion for the United Citizen Federation was a complete success; however, it is unknown whether they captured the planet or withdrew to let the Warrior Bugs do their own thing after capturing the Brain Bug.