Set in a post-apocalyptic world, the game switches between two stories: the first, set three weeks before the events of The Last of Us, follows Ellie as she spends time with her best friend Riley in an abandoned mall in Boston; the second takes place between two chapters of The Last of Us and focuses on Ellie's attempts to scour an abandoned mall in Colorado for medical supplies to heal Joel while dealing with enemies.

The game is played from a third-person perspective; players use firearms, improvised weapons, and stealth to defend against hostile humans and zombie-like creatures infected by a mutated strain of the Cordyceps fungus.

The game received generally favorable reviews by critics, who praised its story, characterization, level design, and depiction of female and LGBT characters.

[2] Throughout the flashback section, players encounter locations and activities around the mall, such as a carousel, photo booth, video arcade, mask store, and water guns.

[3] After an ambush that leaves Joel (Troy Baker) severely injured, Ellie (Ashley Johnson) searches an abandoned mall in Colorado for supplies to heal him.

Riley reveals that she has been assigned to a group of Fireflies in a different city, but broke the rules to come and see Ellie one last time.

Before they part ways, Riley plugs Ellie's Walkman into the sound system of an electronics store, and the two dance on a display case to Etta James's cover of "I Got You Babe".

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.

[6] Riley was chronologically introduced to Ellie in The Last of Us: American Dreams, a comic book written by creative director Neil Druckmann and artist Faith Erin Hicks.

[6] Though initially he only felt that Ellie viewed Riley as an influence, Druckmann later considered her romantic appeal, and decided to explore the concept.

[6] Druckmann felt that omitting the nature of the argument between Ellie and Riley, which took place prior to the events of the game, allowed players to draw their own conclusions.

[3] The smaller time frame of development for Left Behind gave the team an opportunity to trial mechanics and ideas that they were unable to test on the main game.

Tom Mc Shea of GameSpot found the story insightful,[20] and IGN's Colin Moriarty named it one of the game's standout features.

[23] Polygon's Samit Sarkar wrote that Left Behind "serves as a terrific side story" to the main game, but is "even more impressive" when taken on its own merits.

[24] Nick Cowen of Computer and Video Games found the story "less satisfying" than The Last of Us due to its lack of new details about Ellie, but declared it "action-packed and enjoyable" nonetheless.

[20] Helgeson of Game Informer welcomed the addition of Riley, noting that she is "played with the same level of emotional depth and subtlety" as other characters.

[18] The character performances also received praise,[18][19] with IGN's Moriarty and Kotaku's Kirk Hamilton noting that the game improved as a result.

The kiss shared by Ellie and Riley was described by Kotaku's Hamilton as "video gaming's latest breakthrough moment" and "a big deal".

[28] Edward Smith of International Business Times felt that the kiss was "the first example of intimacy in a videogame that's meant anything", recognizing it as "an expression of both burgeoning teen sexuality and of ...

Polygon's Kollar called the game's locations "beautiful",[21] while Eurogamer's Harman wrote that the level design significantly improved the environments.