Refusing to lose another daughter, Joel launches a unilateral rescue mission through the hospital, killing Marlene and several Fireflies to save Ellie.

As they head toward Jackson, Wyoming, Joel lies to Ellie , telling her that there were many others like her and that the Fireflies had stopped looking for a cure. Thematic Significance

The episode opens with a flashback showing Ellie’s mother, Anna (played by Ashley Johnson), giving birth while fighting off an infected attacker. This sequence provides the long-awaited explanation for Ellie’s immunity.

Joel and Ellie finally reach the Firefly hospital in Salt Lake City . There, Marlene reveals that the surgery required to create a vaccine will be fatal for Ellie.