September wants December to steal another initiating reactor from their time because his has lost its charge. Their conversation reveals that all of the 12 Observers on the first surveillance team experienced emotive development by being in this era - and agreed to keep that quiet. Also, they weren't told they were actually scouts for the Observer invasion.
Agent Broyles tells Peter he's on his way; he just needs some things from his office. But Captain Windmark's men go over Broyles's car with the same tech that re-created Nina Sharp's conversation about the child Observer. He knows Broyles is The Dove. Instead of detaining Broyles, Windmark plans to follow him to the fugitives.
The Fringe team finds September at Harvard. He explains that the initiating reactor will provide the energy to start up the device, generating a small wormhole, with the Observer cylinders stabilizing each end. Michael and a guide will then travel to 2167.
While driving, Broyles spots a pair of gloves a Loyalist left on his dashboard. Realizing what Windmark's up to, he informs Olivia. He tries to escape but is captured.
While looking for equipment in the amber, Peter finds a videotape addressed to him. He and Walter watch as Videotape Walter says Peter will receive an unusual letter containing something of Walter's. Then he will learn that Walter has simply vanished - but he'll be alive in the future.
Walter explains he's taking Michael to 2167. Because nature abhors a paradox, it will delete them both at the moment of the invasion. Walter wants to give Peter his life back. As a father, how could he not? They weep, and Walter says, "You are my favorite thing, Peter. My very favorite thing."
At December's apartment, Olivia and Astrid find the Observer dead and the replacement reactor gone. It's now in the hands of Windmark, who's been monitoring the remaining original Observers.
As an alternative to the reactor, Astrid suggests commandeering an Observer shipping lane. They can use her decoded manifest to pinpoint the next one. However, they need that cube-shaped tech to hijack the lane.
Astrid shows Walter something inside the amber: Gene the cow. She says this all will be over soon and they'll be drinking strawberry milkshakes, not even remembering it. He says she always knows how to soothe him. "It's a beautiful name," Walter says. "Astrid."
Windmark and Broyles discuss how some Observers developed irrational emotions. Windmark admits that he, too, feels something: hate. Broyles coolly says the feeling is mutual.
Olivia and Peter flood the Observer HQ's ventilation system with a cocktail of toxins from Walter's cold storage. Wearing gas masks, they procure a cube as Observers and Loyalists die horribly. Olivia sees Broyles on a monitor, and they rescue him.
September has decided he's taking Michael to the future. Peter already knows, but Walter objects. It's HIS destiny. September says when he takes Michael's hand and leads him, the boy will know he loves him. It's not about fate, it's about changing fate - and protecting our children. Walter understands.
It's plan time. Bullets fly as the resistance buys time for the wormhole to be opened. But Windmark pops in and grabs Michael! Peter, Astrid and Olivia are no match for him. Olivia falls to the asphalt . . . and Etta's bullet falls from her pocket. Using her remaining Cortexiphan power, she telekinetically slams an SUV into a pickup - squashing Windmark between them.
September takes Michael's hand, but he is fatally shot. Walter knows what he must do. Peter mouths, "I love you, Dad." Walter smiles and leads Michael through the wormhole . . .
. . . And we're back in the park with Peter, Olivia and little Etta on that sunny day in 2015. But this time, there's no Observer invasion. At home, Peter has mail from Walter: the drawing of the white tulip, Walter's symbol of hope and absolution. September took it from the other timeline and gave it to Walter when he doubted his ability to carry out their plan. Peter gazes at it, puzzled. A thought dawns on him.