Not much is known about the history of Second Lives, other than the fact it was a psychiatric hospital prior to the apocalypse.
Some time during the apocalypse, possibly during the early onset of it, this building was caught on fire as evident with the charred up bodies that are seen around the hospital's parking lot. Eventually Beta and his best friend came across this place when their previous camp was overrun, although it is unknown if his friend was alive or zombified at this point.
In a flashback to 7 years ago, an injured woman wakes up in a crashed car as several walkers start approaching her. She tries to escape but is surrounded and devoured. Among the herd, Alpha walks with a younger Lydia, covered in blood to blend in. When a walker bumps into Lydia, she drops her earmuffs and screams. They are forced to run and seek shelter inside an abandoned hospital, where they find a man wearing a ski mask. Alpha explains to the masked man that she and her daughter don't mean harm. He allows them to stay for the night.
Alpha cleans Lydia in a bathtub. Lydia is scared by the sound of walkers but Alpha insists she just listens to it. They look through supplies and eat. The masked man explains to Alpha that he doesn't want to hear her singing and that the sound of the dead is the only sound he never wants to end. Lydia tells Alpha that she wants to be like her. Alpha is thrilled by this but tells her to stop calling her mama.
Alpha walks around the hallways of the hospital and encounters some walkers. She starts killing them and is joined by the masked man. Afterward, he helps Alpha open the walkers' carcasses so she can use the blood later on. She jokingly decides to call him "B" and claims that makes her the "A". When she tries to see his face under the mask, he demands she leave at sunrise and not come back.
Lydia wanders around the hospital on her own. Alpha wakes up and looks for her. She enters a room and finds photos of Beta with his face scratched out of all of them. Beta finds her and is mad that she wandered there. She then kills a walker wearing a smiley face t-shirt, provoking Beta into a fit of rage. Lydia arrives and ultimately stops him from killing Alpha. He kneels by his friend's side as Alpha approaches him and declares "We are the end of the world." She lifts his mask to look at his face and chuckles, seemingly recognizing him. Beta doesn't want to leave his friend behind and she declares he won't have to, giving him a knife. Beta then proceeds to peel his friend's face off and then he later takes his friend's shirt.