Left Behind is a single player DLC (downloadable content) for The Last of Us. Set as a prequel to the main game, you take control of Ellie a few weeks before the events of the main story. One night while sleeping, Ellie is awoken and startled by her best friend Riley, a young girl who had left the boarding school a period of time ago after a confrontation with Ellie. To make up for leaving Ellie behind, Riley tells Ellie she has a surprise to show her and the two set off to sneak away from the school. As the two venture into a now abandoned Boston mall, events that will forever change everything for the both of them are set in motion.
The game features scenes and gameplay from the end of the Autumn portion of The Last of Us. During these segments, you control Ellie while she scavenges around a different mall to look for supplies to help Joel. You will come across both infected and humans and can interact with them in different ways in order to advance. For example, you can throw a brick near a group of humans to attract the attention of the infected toward them or you can just face them all head on. There are collectible notes and tape recordings scattered around the areas that expand the backstory when collected.
After the initial DLC release for the PS3 and the inclusion for the PS4 in The Last of Us: Remastered, it was eventually also released as a standalone game for both platforms. The standalone versions contains a free trial of the Factions multiplayer game mode of the main game.