Set in a post-apocalyptic world, players control Ellie, a teenage girl who spends time with her best friend Riley after Riley's unexpected return. Naughty Dog began developing The Last of Us: Left Behind following the release of The Last of Us in June 2013, with a team about half the size. Following the decision to create single-player downloadable content for the game, the development team immediately decided that the story would focus on the character of Ellie. They found that players of The Last of Us were interested to learn about events in Ellie's life prior to the events of the main game, particularly the events involving Riley Abel, whom Ellie mentioned in The Last of Us. In addition, they found that some players were interested in the events that occurred between the Fall and Winter segments of the main game, in which Ellie cares for an injured Joel. This led to the team deciding to contrast these two events against each other, feeling it would help the story's pacing. Left Behind was written to focus on the relationship between Ellie and Riley, and to recount the events that defined their later personalities.