In the alternate universe, the Fringe team goes to investigate a breach.  They find a man with tumors all over his face, and he is from our world.  You then see our Olivia and Walter hiding in the background.

36 hours earlier, Walter sees a video of Peter leaving with Walternate to return to his universe.

In a bar, an Observer places a piece of paper near Olivia.  It is a picture of Peter with flames/smoke coming out of his eyes and some sort of spherical machine.  Apparently, Peter is going to be responsible for the end of the world.

The team then heads to Nina Sharp, to question her about the weapons Massive Dynamic is manufacturing and to also see if she can help them cross to the other side.

William Bell has crossed over so many times the atoms in his body have become unstable.  

Olivia has the ability to cross over safely.  With more Cortexiphan subjects, they might be able to cross over without injury to their body.  Broyles has been working with James Heath, Sally Clark, and Nick Lanes, to help them properly control their abilities. With their help, Olivia might be able to save both worlds.

Olivia goes and gives her niece a necklace that her mother gave her before she died.  

The four and Walter cross over by thinking back to when they were children, allowing their imagination to take over.

On the Other Side, Martin Luther King is on the twenty and the Statue of Liberty is made of bronze.  

The other Fringe team goes to meet with the Secretary of Defense, Walternate.  Their Fringe team was founded in 1985 with its primary focus on natural and environmental disasters, created to locate holes on the fabric of the universe.

Peter wakes up and is reunited with his real mother.  He tells her that his other mother committed suicide ten years earlier.  She then gives him Walternate's plans of the machine that he needs help with.

Olivia and Walter go to meet William Bell and they get raided by the other Fringe Team.  Walter takes off running, and the Cortexiphan tests subjects are all killed.  Walter then collapses in front of a hospital after being shot by the other Olivia.

Olivia goes to the other Olivia's apartment.  William Bell finds her there and tells her Walter is in trouble.  

Walternate goes to a room, containing the machine/weapon and the same drawing that the Observer gave to Olivia.