Ok, Walter saw on cctv Peter going with 'bad Walter' to the 'other world', an observer leaves Olivia a piece of paper showing a machine and Peter with weird things coming out of his eyes, she shows it to Walter and he remembers that Peter returning to the other side will cause the destruction of their earth.
At massive dynamic they talk about how when you pass through to the other side your cells don't come back together with the same cohesion, they don't know how William Bell has done it so often but they think he must now be molecularly unstable. Nina says that Olivia can pass over safely using other people who were drugged with the same stuff Olivia was when they were kids.
They are all taken to a new lab which Peter demanded funding for. They have a night off and do different things then all get back together and transfer over which leaves the girl feeling ill and one of the boys covered in tumours.
This is then how the show opened with the 'other world' team of Olivia, Charlie, Broyles and Lee(?) finding there had been a 'breach' in their universe and discovering the tumour covered body. The secretary 'bad Walter' calls the other Fringe team to his office and explains that the body is from an alternate universe and it came with others and they must find them.
Meanwhile Peter is with his mother, coffee is rationed, Walter has left blueprints for Peter to look at.
Both Fringe groups meet at central park, the two remaining 'drugged children' die, and Lee is incinerated but still alive. Walter is shot and gets to a hosipital and collapses.
Good Olivia spies on other olivia who is with her boyfriend, william Bell approaches her and tells her she must trust him and they don't have much time to save Walter.
It ended with 'bad Walter' looking at big machine and taking something out of a case and leaving.
Seriously Blondes, what's not to understand, it's as clear as mud