Olivia is obviously upset about what happened to Peter and Walter is coming over to explain what happened. The Walter in 1985 has a very sick child that is actually dying. He has made a machine in which he can look into the alternate universe. the Walter over there has found the cure but missed it. Walter is going over there to give the cure to the other Peter. But everything not goes as planned. When Nina tries to stop Walter she looses her hand in the machine. The bottle with the cure is crushed and Walter has to find another way to save Peter. So he decides to take him to the other side. But the machine is falling through the ice of Reiden lake together with Walter and Peter. They are saved by one of the bald men. But ofcourse they cannot return now.
Note: in the alternate world the 3 bald guys go to the movies. Back to the future is showing starring Eric Stolz instead of Michael J. Fox.