For the complete The Last of Us: Left Behind experience, read Prima's free walkthrough.
When you gain control of Ellie, you're back in present time, in the mall searching for medical supplies. Before you move, go to the right until you hit a wall and turn left where you will find the Atrium Note that is laying on the ground. Head back to where you spawn to see a locked generator room, learning you need to restore the power to open it. Head to the left, following the cable laying on the ground. Walk through the double doors with the word "Maintenance" above them. You're safe here, so head towards the only direction you can, keeping a close eye out for parts you can use to craft items. Move along the only path you can go, turning left when you come to a dead end. When you enter a flooded room used for shipping and receiving, head to the back where the generator is. You won't be able to start it, but laying on top is the Generator Note, so pick that up. Turn around and pick up the empty gas can laying on the ground.
After you find the empty gas can, you'll encounter four Stalkers. They don't know you're there, so it's best to use Ellie's Listen Mode to pinpoint their locations in the room. Move slowly and don't give them line of sight on you. If you get the chance, use the switchblade melee attack to thin them out. If you do get spotted, use your pistol, but be sure to pop in and out of Listen Mode to keep track of any of the Stalkers remaining. When all four are down, look for a large truck in the middle of the room. On the passenger side, you will be able to siphon the gas you need for the generator. Go back to the generator, fill it with the gas and start it.
To the right and behind the generator is a chain-link gate. Go through it and climb up the ladder. Once you're on top of the air vents, make your way to the left and then forward to the middle of the room. Drop down on top of the large truck and pick up the parts laying on the hood. Head to the back of the truck and jump down, turning right and going straight until you reach a garage door. Use the dolly to pull the skids out, making sure to look in the locker for parts you can use. Next, open the garage door to the right of the first, also checking for parts. Position the skids under the open garage door, then climb up the boxes to the left, making your way back on top of the air vents. Follow them to the end of the room, drop down and open up the double doors to enter a room with five Infected, including a Clicker. Turn off your flashlight and make use of your Listen Mode. Stick to the right side of the room and sneak all the way to the end, knifing the Infected next to the door. Don't turn to fight, just open the door and get out of there.
You're back in the room where you originally started the chapter, only this time there are two Clickers in there with you. Don't try to fight them, just use Listen Mode and avoid them. You need to get access to the gate in the middle of the room. If there's a Clicker near it, throw a brick to distract them. If you are one who prefers chaos, craft a Molotov Cocktail and throw it at one of the Clickers. Usually the sound will attract others in the area, giving you the multi-kill. Go through the gate, head up the escalator and turn left. Head into the tent and grab the Atrium Recorder laying on the ground.
If you left the Clickers alive, they are most certainly following you. Go to the right of the escalator and climb up on the collapsed roof before jumping over to the helicopter, ending the chapter.