Walternate gets word that someone very similar to him has been admitted into the hospital.  

Olivia and William Bell arrive at the hospital looking for Walter.  The other Olivia arrives shortly after them.  Bell is able to distract her just in time for our Olivia and Walter to escape.  The other Olivia sees the hospital surveillance video, showing Walter and a woman that looks like her.

Walternate explains to Peter that when Walter entered their world it created a lot of problems on their side.  The laws of physics were turned into mere suggestions.  The machine Walternate is trying to create will somehow fix the problems in their universe.

Walternate then tells the other Olivia that their doubles cannot be trusted and that they were responsible for his son’s disappearance twenty years ago.  

Driving through a dead forest to his lab at Harvard University, Bell tells Walter that he caused all the destruction around them when he took Peter.

William Bell returned to the other side to prevent a war and to stop a chain of events that was sent in motion the day Walter stole Peter.  Bell also created the shape shifters, to keep tabs of Walternate by working for him.  

Walter and Bell build a door stop, to reopen the crack that Olivia can create between universes.  

Olivia goes to the other Olivia for help in locating Peter.  They get into a fight and our Olivia wins.  She then dies her hair and goes undercover as her other self.

Peter begins working on part the machine his father gave him.  It appears to be symbiotic in nature, needing some sort of organic interface to work.  The machine seems to only respond to him.

Olivia shows Peter the picture the Observer gave her, showing that Walternate is trying to destroy their world.  Olivia tells Peter that he belongs with her.  After a passionate kiss, Peter decides to return to our world with Olivia.  

Walter is able to reopen the door to our world with Bell’s help. 

Bell tells Walter that he took out part his brain because Walter asked him to.  Walter did not like what he was becoming.  The three make it back home.  

Turns out, the other Olivia was the one that crossed into our universe.  She somehow blows up a hand grenade and switches places with our Olivia.  

The other Olivia contacts Walternate using a special typewriter and our Olivia is locked up in their world.