"5-20-10" was an almost-forgotten combination used previously in the episode "Jacksonville." The combination was to a safe protecting a very important device. In the "Fringe" episode, "Five-Twenty-Ten." Walter's (John Noble) brain has been through the ringer since the beginning of the series, and he almost lost the important piece of information needed to get the next part of The Plan from the safe.
Now, Peter (Joshua Jackson) is putting his own brain through a similar ordeal after having implanted Observer tech in the base of his head. Only Walter wishes to be "himself" again. Peter wants the pain of his daughter's death to go away. He also wants to avenge her death without care of the cost.
Peter creates his own plan
Walter's plan to save the world from the Observers is a mere afterthought to Peter, who is trying to work far ahead of the team on his own plan. The Observer tech he implanted in his own brain is allowing him to see the future.
Viewers have no idea what Peter is doing at the beginning of the episode, other than observing various Observers. It is only when the scene is repeated that we realize Peter can see the future. He is planning something, and it's impossible to predict what the plan may be until it unfolds. Just as with Walter's plan, viewers are along for a wild ride.
Finally, a use for William Bell's hand!
The team extracts the next Betamax tape from amber and learns that the next part of The Plan is hidden in William Bell's private storage. They need "Belly's" hand print to get in, which means there was a reason Walter cut it off Belly's body while it was in amber. Walter just couldn't remember why he needed it until now.
Before they can get in, however, they need to clear away the rubble piled in front of the building. For that task, they need another blast from the past: Nina Sharp (Blair Brown). Nina looks almost as good as always, despite her advanced age. Nina gives them the disintegration tool, which they use to disintegrate the rubble and enter the building. The sight of Walter raising Belly's hand up to open the storage area was quite a sight. Walter has a couple more mini freak-outs involving a Bowie record Belly stole from him and the safe initially not meeting Walter's expectations. Finally, they take what they went there to get and leave quickly.
Peter learns how to see the future, accounting for variables
Peter makes a mistake in his initial plan, which viewers still aren't privy to until later in the episode. He realizes that he needs to account for some variables. Once he gets the hang of it, he enlists Anil (Shaun Smyth) to help him with his side plan. Peter's plan goes well, culminating in a flesh-eating toxin-bomb leveled on three of Windmark's (Michael Kopsa) top lieutenants. The spectacular jaw-splitting, face-melting sight is disgusting and really fun to watch (is that wrong?).
Walter wants to be his new-old self again
Walter is very cruel to Nina at one point, coldly telling her that William Bell never loved her. Nina feels that Walter will soon become just like Bell, God complex and all. Walter feels that Peter will help him keep his humanity, but Nina doesn't think that's enough. However, Walter finds a picture of Nina in Belly's safe and realizes that Belly loved Nina after all. Walter brings the photo to Nina, along with a request: He wants her to remove the pieces of his brain that had been replaced, so he can go back to being "socially awkward but nice" Walter.
Peter loses his humanity to Observer tech
After Peter kills the group of Observers, he coldly tells Olivia (Anna Torv) what he has done. He simply states -- in Observer-like, unemotional fashion -- that he used the technology from the team's first "Fringe experience" to give the three Observers their last Fringe experience. Peter tells her flatly that Windmark will be his next victim.
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