It's daylight now, at Harvard, and Walter tells a zombie-like Ella that the diagnostics are almost complete. Then he asks her if she remembers how she used to call him "Uncle Walter." Not the time, Walter! She says simply that she doesn't remember much from before "it got bad." So Walter gives a speech about how he's sorry and that he understands that words can't come close to mitigating her loss. He says -- much like Peter told Walternate -- that if he could go back in time and change things, he would. "I would give anything to be able to go back and make different choices."
Sounding like she herself blames Walter somewhat for what happened, Ella interrupts to tell him that he can't. "There aren't any happy endings nowadays, are there?" she says, and he meekly agrees. Seeing his anguish, she softens and says she remembers the cow that used to be there. "She had kind eyes," she says, and Walter agrees, smiling. "My Gene. I do miss her," says Walter, and because we're at the three-quarter mark it's time for Walter to have that epiphany that gives him a solution. A beep on his computer and a glance at the computer screen, and then Walter says, "That's it!"
Peter is standing looking out a window of his house. He hasn't changed his clothes. I'd really like to know if he made it through the second bottle of vodka. Anyone can make it through one. The doorbell rings, and it's Walter, under armed guard, clutching papers in his hand. He tells Peter that it's not too late, that Peter can save both worlds. Inside, he shows him the diagrams, and tells Peter he simply has to make a different choice, and should something go wrong, then Olivia will be their fail-safe.
"Walter, stop. Olivia is dead," Peter says, and Walter says, "But she won't be. Not then." Peter looks at the diagrams Walter has of the machine and points out that he turned it on fifteen years ago. Walter excitedly says all the time he sat in prison, he couldn't figure out where it came from. "I knew the pieces were buried millions of years ago, but how did they get there, so deep in the past?" he says, before supplying the answer: he sent them there, via the wormhole in Central Park. "Peter, you can stop the destruction before it occurs!"