Back in a Flash is the first chapter of Left Behind. After a short flashback to Riley's surprise visit at Ellie's school in Boston, you play Ellie in the snowed-in Colorado Mountain Plaza, searching the mall for anything that can help Joel.
Part 1 of 6 in Left Behind
The chapter starts with the end of Joel and Ellie's escape from the university on Callus. It then flashes back to summer in Boston: Riley sneaks into Ellie's dorm, pretends to be infected and gets a switchblade pointed at her. She reveals she has joined the Fireflies and talks Ellie into skipping drills to go exploring.
Back in the present, Ellie has hidden Joel and Callus in an empty yogurt shop. She binds his wound with tape and her summer shirt, then locks them inside with the shutter and heads off alone.
Infected can be heard somewhere in the mall. The pharmacy has been picked clean, but through the window of a locked back room Ellie sees a first aid kit on the floor. A note gives the combination to the American Princess store next door and explains the pharmacist was locked in there after attacking someone. Inside, spores fill the air, and the pharmacist's body has the key and a photo of a couple. On the way out, stab the passing clicker and take the bottle it drops.
The kit in the back room turns out to be empty. Ellie spots a Black Hawk that has crashed through the roof outside the pharmacy, so you cut through a wrecked salon with a dead soldier, a note and a health kit she keeps for herself. The chapter ends as she vaults a railing on the stairs heading for the wreck.
The Enemy of My Enemy
The Enemy of My Enemy is the fifth chapter of Left Behind and its main combat chapter. With the medical kit in her pack, Ellie fights back across the Colorado Mountain Plaza to Joel's hiding place while armed survivors and infected roam the halls, and you can turn the two groups on each other.
So Close
So Close is the third chapter of Left Behind, back in the present at the Colorado Mountain Plaza. Ellie has to restore power to reach the crashed military helicopter on the mall's upper level, which means siphoning gas for a generator, fighting stalkers in a flooded stockroom and sneaking through a pet store full of infected.
Escape from Liberty Gardens
Escape from Liberty Gardens is the final chapter of Left Behind: Ellie and Riley run from a horde of runners through the Boston mall, climbing scaffolding to find a way out. It has no collectibles, and finishing it unlocks the gold trophy Don't Go.
The University
The University sends Joel and Ellie on horseback across the University of Eastern Colorado near Boulder, looking for the science building where Tommy says the Fireflies are based. It mixes quiet exploration with a dorm full of infected and a fight against armed looters inside the lab building.