In Wyoming, months after Henry and Sam's deaths, Joel tells Ellie about his brother Tommy joining and quitting the Fireflies, and how they got separated on bad terms. She wonders if he will help them, to which Joel responds "I guess we'll find out". They go past a dam, and encounter a small grave with a teddy bear on it, that makes Ellie think about Sam and his robot toy. Joel refuses to talk about it, and tell her to just move on with her life. They eventually reach the closed gate of a power plant, and are held at gunpoint by several people. Tommy is among them, and greets his brother with a hug. He introduces them to Maria, his wife, who along with her father is in charge of a nearby self-sustained settlement of people, and are using the power plant for electricity. Ellie talks to Maria about how she learned to ride a horse after noticing one outside of the plant, prompting Joel to ask her who taught her. As he shows Joel around, Tommy says he went back to Texas a few months back, and tries to give Joel a picture of him and Sarah together he found at his house, but Joel refuses. The brothers later witness the workers getting the power generator up and running again.
In private, Joel tells Tommy about Ellie's immunity and asks him to bring her to the Fireflies, which Tommy refuses, since he is now responsible for all the people of his community. Angered, Joel shoves him and calls him ungrateful for all the years he kept the two alive, but Tommy replies that he "got nothing but nightmares from those years". As he says this, the plant is attacked by bandits, forcing Joel to help Tommy fight them. After eliminating the bandits, Tommy (seeing Joel and Ellie interact) agrees to take her, but Ellie, suspecting that Joel is going to leave her, steals one of their horses and runs off into the woods.
After fighting more bandits, Joel and Tommy track her down to an abandoned ranch house, where Ellie confronts him about dumping her on Tommy. She brings up Sarah, saying that she is not his daughter and they are doing fine together, which just seems to confirm his decision. More bandits arrive at the ranch house and the brothers manage to kill them all. However, as they are heading back, Joel has a change of heart and decides to go with her himself. Tommy informs them that the Fireflies are in the Science Building of the University of Eastern Colorado, and Joel bids him goodbye.
"Tommy's Dam" contains the following collectibles:
- In the cinematic trailer, Ellie is holding a regular 9mm pistol at the dam, but in-game, she holds her unique Beretta pistol.
- In the ranch house in the upstairs room with the computer there is a copy of the real world board game on the window sill.
- The ranch house is very clean and orderly in the game, with intact furniture and windows. However, the concept art shows a much more untidy ranch house with smashed windows.
- This chapter and "The Firefly Lab" are the only chapters without any infected enemies.
- in the very beginning of the chapter before the player reaches the dam, there is a flock of birds that flies by over the river. If the player turns around and watches those birds fly away they will either just disappear or go through some of the terrain
- When the player reaches the dam and makes the bridges, Ellie will turn her wheel faster than Joel will turn his. Even if Joel begins first, Ellie will beat him every time, despite Joel being much stronger than Ellie
- At this same part, the dam is designed to force the player to make their own bridge. On the right behind the shiv door, there are mounting brackets on the wall that would indicate a walkway of some sort fell down, but it doesn't connect to anything, so there would be no way for a worker to get to the area where the shiv door is without using the bridge that Joel and Ellie make.
- When you arrive at the ranch house, the horses change. Tommy's horse is a paint when they arrive, and when entering the house there are two paint horses and Callus. When they have killed the bandits, each horse is now unique, and have changed positions.