As Bellivia continues to be a combination of both a very weak Olivia and a very strong William Bell. When their experiment fails and Bell suggests she only has a day to stay alive, the team goes into overdrive trying to get her back. By taking acid.
Her fears of being lost in her own subconscious are causing her to hide, making it virtually impossible to find her. They had hoped she had left a trail to be found, but instead she created a world where only she would know where she felt the safest.
Peter imagines the last place anyone would look for her would be Jacksonville, and she would think that a good place to hide. A swarm of zombies try to stop them, but they hope onto a looming Zeppelin to get there.
Peter finds Olivia, disguised as her childhood self. After being chased, Peter dying and facing her stepfather and an army full of men, little Olivia puts out her hand, tells them to stop. She is not afraid of them. At that moment, Olivia becomes an adult. William Bell points out that she just faced her fears, and has always had it in her to be as strong as he and Walter always knew she was.
The experiments don't work, and Bell is lost forever, leaving behind a stronger more courageous Olivia Dunham in his wake.