Joel wakes up to find himself and Ellie alive on the riverbank, along with Sam and Henry. Henry is glad that Joel is okay though Joel is less than happy that the brothers left him and Ellie to die in the city. Henry manages to defuse the situation that if the roles were reversed Joel would do the same, and that he saved them from drowning. Joel slowly acquiesces and the four went on their way. Along the beach they come across a ruined, weathered boat, as well as a note from Ish, one of the survivors after the outbreak. The four find a large pipe, covered with a grill, located on the cliff side. Henry assures the group the sewer should go all the way through to the radio tower. Immediately the group reaches a fork as the pipe opens up into the sewer; Henry and Sam go left while Joel and Ellie head right. Joel and Ellie find a small area enclosed with a wire fence, and a blocked door. Joel pulls a grate off the left side of the wall for Ellie to crawl through, and open the door. Ellie then wonders if the others will join them to Tommy's. After exiting this area, they walk straight ahead to find the others who have found a path.
The four arrive in a room filled with water; Sam exclaims he cannot swim. Ellie points out the generator across the room whilst Henry notices an old platform on a motorized track. After a momentary detour to both open the gate, and to guide Ellie to the generator using a pallet, as well as an encounter with two sleeping Clickers Henry, Sam, and Joel are then brought to the other side.

Following the corridor, they arrive at a set of double doors painted like a castle in a very bright, cheery fashion. Upon opening it, Joel sets off a 'sound trap', which, according to Henry, is a type of alarm. The sewer appears to have been a settlement, a long time ago. As a momentary reprieve, Ellie and Sam play with a discarded soccer ball, where Henry gently warns Sam to 'keep it down'.

Soon, the group hears a herd of Infected coming their way; they quickly dispatch them. Joel observes that since a Clicker was part of the horde, the Infected have been in the sewers for a while. Furthermore, in one of the many rooms, there are three skeletons covered in cloth. Scrawled on the floor are the words, 'they didn't suffer.' Reading the accompanying note reveals one of the adult caretakers had killed the children, to avoid the possibility of them becoming Runners. The room is filled with children's toys. (Later you find out that this is Kyle who shoots the kids.)

Continuing on, they walk down a flight of stairs leading to an open space. The ceiling has fallen away on the left side, but is too high to reach. Joel goes to open the wire door on the right, but unknowingly triggers a safety door. It slams down, splitting the group into a parallel Henry with Ellie, and Sam with Joel. They briefly talk before numerous Clickers, alerted by the noise, chase away Henry and Ellie. Joel and Sam continue on, but not before dispatching a large group of Stalkers.

They wind up in what could have been the main area of the survivors' hideaway. There were areas for anything and everything: sleeping, eating, washing, toileting, schooling, and storage. The little rooms all had small, different personalized touches. Reaching the far side, through what looked to be a nursery, Joel boosts Sam up a small wall to drop down a ladder. Running through a corridor to then drop down into another room, they regroup with Henry and Ellie there is no time for pleasantries however, as they must flee from an incoming herd of Infected.

They run into another area, and Joel slams the door shut and barricades it. When confronted with another blocked gate, Joel and Henry start to utilize their strength to try and force it open, before Sam crawls through a grate. Ignoring Henry's panicked shout, Sam quickly opens the gate. They run along the path until ending up in a decrepit locker room, with their way out blocked from the outside. Henry boosts Ellie and Sam through a small window over the door, and stays to help Joel fend off the herd of Infected. Soon Ellie shouts out that the door has been opened Henry and Joel squeeze through and out. As the group revels in the fresh air, Sam points out a warning on the wall, declaring the fact Infected were inside.

Joel then asks how far the radio tower was; Henry replies that it was close. The group then walks down the path, where they see the radio tower in the distance.

The group continues through the suburban neighborhood, noting the sights, and explaining long dead things to the kids. It's a general moment of respite, with the kids playing, exploring, and taking note of a Firefly insignia, indicating they had passed through. Whilst trying to make their way through another part of the neighborhood to the radio tower, they are ambushed by a sniper who, taking refuge in a house down the street, takes potshots at the group. Joel readily volunteers to act as both distraction, and subduer. Moving towards the sniper while simultaneously taking cover, and gunning down the sniper's allies, Joel reaches the house. He is taken by surprise when the sniper jumps him, shiv in hand they briefly engage in a power struggle, before Joel gains the upper hand. Killing the sniper, he notices the group moving up the road, and moves to cover them via the mounted sniper rifle.
More Hunters emerge from the woodwork, but are quickly dispatched by Joel. Unfortunately, the Humvee from Pittsburgh makes its appearance, rolling down the street towards Henry, Ellie, and Sam. Joel's shots are ineffective, until a Hunter pops up to throw a Molotov at Henry and the kids. After Joel snipes him, the Hunter drops the Molotov and it immolates the Humvee's interior. The vehicle then crashes into a nearby house.

The group seemingly safe and collected, are caught off guard when a few Infected, attracted by the sounds of the recent fight, rush them and manage to tackle Sam and Henry to the ground. With help from Joel, they manage to kill them, before they caused any serious harm. Soon, an enormous herd of Infected emerge in droves, prompting Joel and the others to slip out through a gap in the wooden fence behind the house, and escape the Suburbs.

They finally arrive at the tower, and take a temporary reprieve. Henry and Joel bond over stories of Joel and Tommy renting a pair of Harleys to drive cross country. After Ellie excuses herself to visit Sam, who is taking inventory of the food they had found that day, Henry reluctantly faces the fact no one from his original group will come.

Meanwhile, Ellie and Sam converse about their fears, their accomplishments, and the existential possibility of an Infected still being human, deep down. At first, Ellie uses humor to deflect Sam's probing queries, but eventually responds accordingly. Ellie admits she is afraid of being left alone, and Sam is scared of being infected. Sam tells Ellie that his brother once said, that the Infected were no longer 'people'; that they've 'moved on'. Ellie agrees, telling Sam that that person is 'not in there anymore'. Before Ellie bids Sam goodnight, she remembers to give him the toy robot he was fiddling with back in Pittsburgh. With her parting words, 'if [Henry] doesn't know about it, he can't take it away,' she leaves for bed. After a moment, Sam drops the toy to the ground, and pulls up his pant leg to reveal an infected scratch.

The following morning, Ellie wakes up to Henry cooking breakfast. She asks where Sam is, and Henry replies that he let him sleep in 'for once'. Henry suggests she go wake him, and she readily agrees. However, moments after she enters the room she is attacked by Sam, who has turned overnight. Panicked, Joel searches his bag for his gun, but Henry deters him with a stray round from his own gun.

Henry hesitates, prompting Joel to reach for his weapon again. No sooner than he does, Henry shoots his brother in the head. Distraught over his actions, Henry blames Joel for what's happened to Sam and commits suicide, shooting himself in the head.