The Plot
In Gears of War 2, the Locust have returned stronger than before. Humanity hoped that the Lightmass Bomb would end the war by destroying the Locust deep underground where they lived, however, the Lightmass Bomb accomplished little beyond wiping out the Kryll breeding grounds, making travel at night somewhat safer.
The single player campaign begins out with Marcus Fenix and Dominic Santiago of Delta Squad being joined by a new member, Benjamin Carmine. Carmine is the brother of Anthony Carmine, a soldier that died early in the first game at the hands of an unseen sniper, a fact that Dominic (Dom) makes direct reference to more than once. Marcus and Dom train the rook which serves as a training sequence for the player. Afterward, they are tasked to defend the hospital they are stationed at from a Locust attack. Along their way through the hospital, they meet up with another Gear named Tai. Tai has earned the nickname "Bulletproof," and maintains a very spiritual approach to battle. They manage to clear out the hospital and are then informed that they are to take part in a counteroffensive against the Locust.
Dom, however, is far more concerned for the safety of his wife than in the battle ahead. Anya, the intel supervisor, informs him that she might have found a lead on Maria but that it's unlikely she's still alive. Dom demands to know anyway and Anya informs him that his wife may have joined with the Stranded, humans that are without support in the heart of Locust territory. Afterward, the crew meets up with Dizzy, a Stranded that has joined the Coalition of Organized Governments, or COG. Dizzy drives a Derrick, one of several mobile platforms used for drilling that play a key role in the upcoming operation. Used as a troop transport that he has nicknamed Betty, Dizzy follows the convoy out to the strike zone where they are expected to launch their offensive.
Along the way, they take on Reavers and Brumaks, beasts of burden used by the Locust as air and heavy ground support, respectively. They eventually arrive at Landown, the designated strike area for the Derricks, and fight their way through to clear it out and make it safe for the operation to begin. As they prepare to load themselves into the grind lifts for insertion underground, an unknown Locust General appears and wreaks havoc amongst their ranks. Being already strapped into pods, Marcus and Dom are forced to watch helplessly as the pod begins the launch sequence and hurtles them away. Deep underground, they find themselves in the Hollow, the home of the Locust. After fighting their way through several caverns and watching as several more pods drop from the surface, they eventually discover the Locust's new weapon: a titanic worm that is boring tunnels large enough to weaken and collapse the foundation of cities on the surface, dropping them down.
Eventually ambushed and outgunned, Marcus and Dom make contact with Private Augustus Cole, a teammate from the previous game. They continue along to find another friend, Damon Baird, the demolitions and intelligence expert. They free him from the Locust storage canister that he was locked in and he tells them the Locust were intending to "process" him, though even he isn't sure what that means as the Locust were never known to have taken prisoners. They soon find out more when they board a Locust Torture Barge intending to take it deeper into the Hollow.
The Barge is a blimp-like vehicle that the Locust use to torture humans. Marcus finds Tai in one of the cells, tortured and scarred both physically and mentally. Marcus hands him a shotgun to help fend off locust that are converging on their position, but watches in horror when Tai turns the gun on himself and pulls the trigger underneath his jaw.
They witness the complete destruction of a city from the surface as it falls into the crater created by the giant worm's digging and are forced to make their way through the still burning ruins in the hopes of finding a way out. They eventually make it to high ground and contact command who send in a chopper rescue squad to extract the team. As they fight their way to the chopper and begin to lift off, the giant worm erupts from the ground and rises towards them. Carmine falls out of the chopper and into the beast's mouth before it swallows the chopper as well.
Since the beast swallowed them whole, the other COGs remain together and in one piece inside the worm. They decide to take it out and look for Carmine. Avoiding stomach acid, digestive teeth, and nearly being crushed by a giant sphere of debris, they discover Carmine fending off a swarm of creatures. Though the creatures are scared off, Carmine has already suffered grievous injuries. Carmine says his final words to Marcus: "Sarge?..(Marcus replies "Carmine..." sadly) I hurt sarge...tell my brothers... tell my ma... tell em I love em, and...and.." and dies. Marcus Fenix collects his COG tag, number 838186-22AO3-SF and a letter that he had wrote to his older brother Clay, to deliver both to Carmine's family.
After Carmine dies, Marcus and the others make their way deeper into the worm, discover its hearts, and sever their giant arteries with a chainsaw bypass, killing the beast. The beast lurches to the surface and collapses as it dies. Delta Squad cut their way through the skin of the worm and are disgorged on a wave of blood nauseating everyone, especially Baird.
Marcus orders a Raven to extract them but is informed instead that they are tasked with a new mission. The incoming Raven drops off a Centaur, a tank with monster wheels. Colonel Hoffman explains to Delta Squad that they must investigate an abandoned outpost recently discovered by the COGs. Though Marcus feels that they've done enough for now, Hoffman insists leaving Delta Squad with little choice. They arrive at the outpost where Marcus orders Cole and Baird to guard the Centaur while he and Dom go inside to investigate.
They find a large security door and a mysterious figure who shows himself on the screens tells them that they need proper identification to enter. Marcus doesn't play around and simply orders Baird to build a large bomb to blow the door open. After fighting off some stray wretches, they return to the Centaur where Baird has the bomb waiting for them. They carrying the bomb back to the door and successfully blast it open. Marcus and Dom head deeper into the facility and make a shocking discovery: Locust-like creatures in glass tubes, apparently mutated from humans. They also discover that the base is abandoned and that the face that has been taunting them is actually an AI that is continuing to run the facility. After finding the data they need, they trip an unexpected system that begins to re-awaken what is in the tubes.
The half-human, half-Locust creatures, dubbed "Sires" by the AI, begin to break out of their containment tubes. Marcus and Dom are forced to fight their way out only to hear from Baird that the bad weather has turned into razor hail, a deadly hail made of ice shards. Using cover as much as possible, they manage to fight their way back to the Centaur where they protect Baird as he fixes the tank. After fighting off a small swarm of Reavers, Baird fixes the Centaur. Delta takes off, leaving the lab behind and heading for the "backdoor" into the Hollow where the Locust Queen may be according to the recovered data.
Reporting their findings to command, Delta Squad is then assigned to take the Centaur and immediately journey to this location in an attempt to take down the Locust Queen and end the war. After crossing frozen lakes and fending off Reaver and Locust attacks on their way up the mountain, Delta Squad arrive at the entrance. They discover a group of Stranded hiding in the caverns there with Franklin, the gas station owner from the last game, among them. He has given up the station in favor of an underground hideout. Marcus orders Cole and Baird to escort the stranded to the surface using the Centaur, but not before Dom asks Franklin if he has seen Maria. Surprisingly, Franklin says that he has, but then says that he thinks he saw her being taken deeper into the stronghold reducing her chances of survival. Still, he gives them a route to a camp where she might be. After that, Delta splits up and Marcus and Dom head deeper into the mountain toward another location that Franklin had mentioned: Nexus, the central stronghold of the Locust.
Fighting their way into the Hollow beneath the mountain, the friends arrive at the Locust highway. Franklin had told Dom he should branch off here to find Maria, but Marcus thinks they should focus on the mission. Dom reminds Marcus that he abandoned his post, the crime for which he was found in prison at the beginning of the first Gears of War, to try and save his father. Marcus is forced to acquiesce when he realizes that Dom is simply trying to do the same thing.
They come across a lake and although the boat they find is already half submerged in the water, and thus useless, the pier they are on breaks free and becomes a makeshift raft. After fending off several assaults, they manage to board one of the Locust gunboats and use it to fight their way further in until they are pulled over a waterfall. Surviving their fall, they believe themselves safe until they are attacked by the Leviathan, a massive water creature that tries to swallow them. Marcus fights it off by lobbing grenades into its mouth, eventually killing it.
After getting to shore and pushing further into the Hollow, they discover a Locust terminal. Marcus orders Jack, their cloaked intelligent robot to do a facial search for Maria. The terminal reveals a symbol linked to a section of the prison camp. Either by fighting their way through or stealthily avoiding the patrols, they eventually find Maria. At first Dom sees what he wants to: his beautiful wife returned to him in perfect health. Marcus, however, breaks out of his delusion and he sees her as she truly is: starved nearly to death and so psychologically broken she seems unaware of his presence. Marcus leaves Dom to whatever fate he decides is best. In the end, he puts his wife out of her misery, vowing to exterminate every single last Locust.
The two Gears head deeper they eventually arrive at Nexus where the Locust Queen commands her legions. Once they find a safe spot, they order Jack to set up a beacon signal so that the other Gears know where to drop down with their grind lifts.
Cole and Baird drop down directly over Dom and Marcus, and Delta squad is reunited. Together, they head towards the Queen. Along the way, however, they start noticing Locust fighting amongst themselves. The Queen is heard broadcasting propaganda which reveals what is going on: it appears that the Locust have been fighting Lambent (Locust infected by exposure to Imulsion) versions of their race all this time, a civil war that has divided the Locust forces and explains their desperate raids against the human settlements. This would also explain why the Gears had a relatively easy time getting inside the lair.
They get ever closer to the queen until they find a recording from Marcus' father, Adam Fenix, telling them they should sink Jacinto themselves, as it would fill the Hollow with seawater and drown everything inside. Marcus considers it eventually telling Control they should do it. He transmits the data so that Control can use them to come up with an appropriate plan on their end. The data secured, the four fight their way to the Locust Queen's throne room. When they arrive, they discover that the queen is oddly human in appearance with Cole remarking that she's supposed to be hideous. She is joined by Skorge, the same Locust General that they had seen on the surface. The queen makes cryptic references to Marcus in saying that he is not following in his father's footsteps, implying she knew him, and leaves Skorge to deal with Delta Squad. Marcus orders Cole and Baird to pursue her, while he and Dom face off against the mighty Skorge.
Skorge, wielding staff with chainsaws on both ends, duels with Marcus who eventually manages to push him back. Skorge flees on a larger version of a Reaver, called the Hydra, but Marcus and Dom pursue him. They find Cole and Baird who inform Marcus that the queen had escaped. Marcus orders them to follow suit and head outside the palace to a stable where a a pair of unmanned Reavers lie in wait. Hopping on and taking control of them, they race through the caverns and eventually across the surface as they head towards Jacinto to begin the steps needed to sink it. On the way out, Skorge pursues them but is successfully fended off and presumed dead. Afterward, Marcus receives a distress call from Anya telling them the Locust have caught wind of their plan to drown them first and are launching an assault on the city to stop them. Delta arrives at Jacinto after nearly being shot down by their own forces for riding a Reaver and with Hoffman's help, buy time by fighting off the Locust assault.
Eventually, they hear about a sinkhole inside the city and are ordered to head in to clear it out so that Ravens can drop a Lightmass Bomb into it to sink Jacinto. They fight through the ruins of Jacinto around the sinkhole and eventually manage to hijack a Brumak by killing its driver. Using the Brumak's superior firepower, they journey into the sinkhole and lay waste to the Locust hordes that stand in their way. They eventually enter a vast cavern and blast the roof supports, dropping the ceiling and allowing the Ravens to come down and drop the Lightmass Bomb. However, the Brumak that they are on begins mutating from standing in the Imulsion that the cavern is filled with. Marcus and Dom abandon the Brumak by getting picked up by Baird and Cole on a Raven. The now-crazed Brumak smashes the helicopter carrying the Bomb out of the air and into the Imulsion. Without the Bomb, there is no way to sink Jacinto. However, Marcus realizes that if the Lambent Brumak explodes, it may generate enough power to substitute itself for the Lightmass Bomb. Using the Hammer of Dawn, Marcus targets the Brumak with the satellite beam weapon, killing the Lambent Brumak and setting off the explosion that tears apart the foundations of Jacinto. Seawater rushes into the sunken hole, flooding the Hollow and presumably ending the war.
However, as the final scenes play out in following the helicopter exodus from Jacinto, the Locust Queen's voice can be overheard talking about "unintended consequences".
Following the credits, a final voice recording of Adam Fenix plays asking people if they can hear him, and then proclaiming: "What have you done?!".