Walter's up early, cooking up something green to inject into his ass. He claims it's going to make him smarter and fill in the blanks that William Bell removed from his brain.
Great Scott! It's Christopher Lloyd! He's an old dude in a nursing home, visited in the middle of the night by his dead-for-25-years son. The son was sent by an Observer to give him a message, but we don't know what the message was because Christopher Lloyd was sleepwalking at the time and doesn't remember. So it's up to The Team to help him remember.
Meanwhile, The Observer's in Brookline, stopping a jewelry store robbery, complete with catching bullets, kicking the robbers' asses, and saving an asthmatic woman.
Walter uses hypnotherapy and some kind of aura-reading glasses to work on Christopher Lloyd. His son told him that he would meet Walter Bishop and that Christopher Lloyd would help him.
The Observer pops by once everyone else has gone off investigating, asking to speak with Walter. Walter wants to know how to save Peter from the Doomsday Machine, but The Observer tells him that there are many possible futures and he can't help. Instead, The Observer asks Walter for help and gives him this message: "Give him the keys and save the girl." He also tells him about a chain reaction of events - Kid Peter caught a firefly, so a little girl didn't, so she stayed out too late, so her dad went looking for her in the rain, and he skidded and killed a pedestrian - and the pedestrian was Christopher Lloyd's dead son.
As Peter and Olivia are coming back with the chick from the robbery, The Observer causes a car accident, making her go all asthmatic again. (But he left the store with her inhaler.) Walter shows up and Peter says, "Give me the keys and save the girl." Walter's worried that this whole thing has been a chain reaction to kill Peter and restore the balance Walter disrupted when he took him.
While Walter rigs a homemade inhaler out of a soda bottle and a pipe, Peter finds The Observer. The Observer uses his pulse gun to knock Peter out, after Peter asks what's going to happen to him and the whole Doomsday Machine thing.
The Observer disappears, Peter survives, and now everyone's confused as hell as to what The Observer is up to, if it wasn't killing Peter. Once Peter gets back to the lab, he grabs some milk from the fridge - only it's not just milk, it's got Walter's brain-building concoction in it. Peter falls to the floor in seizures. Olivia calls Walter and gets instructions on what to inject Peter with to get the seizures to stop. Walter thinks The Observer was there to save Walter's life - to prevent him from drinking the milk, which surely would have killed him.
In the end, we find out it was all a test - to make sure Walter was willing to let Peter die. And now that they know he was, The Observers say he will be willing to do it next time. Not sure what that all means, but I do love a good Observer/chain reaction story.
Walter's Food Thing of the Week: When high and injecting himself with homemade fluids, Walter orders pizza at 2 in the morning. Also, Walter and Christopher Lloyd bond over their mutual love of strawberry milkshakes.
This Week's Code: UNITES.