Ellie, one of the main protagonists, is immune to the infection that had spread across the US. The military keeps all the uninfected civilians in quarantine zones under their watch, while an anti-military group, The Fireflies, uses infected people for experiments in trying to find a cure. With Ellie being immune, they chase after her, and she willingly goes with them in hopes of stopping the plague. Joel, the main protagonists, grows attached to her, especially after the loss of his daughter. He goes to save her from the experiment, which will kill her in the process.
(Read after watching)
Joel lied to her when she asked if everything he said was true. They never did stop looking for a cure, and he doesn't know if there are dozens like Ellie. He is just too attached to her to ever lose her, like he lost his daughter Sarah. You can see the look of resent in Ellie's eyes, as if she doesn't believe him.
