The Fringe gang go out to visit their doomsday device that Massive Dynamic is building which I’m sure is not at all in danger of ever almost going off in some future episode, and the machine (and all the equipment around it) goes a little haywire on Peter, because it has a bit of a crush on him. Walter, not satisfied with almost killing his son last episode, goes in search of some of his DNA that Belly kept around after removing parts of Walter’s brain, so he can regrow his grey matter, only he winds up snorting some chimpanzee DNA instead. That makes more sense if you’ve seen the episode. I think.
Astrid (and later Olivia) go through Fauxlivia’s diaries partly to try to figure out why dead shapeshifter bodies are turning up, and the working theory is that Walternate is tying up the loose ends of the infiltration operation, but it becomes clear that there has to be someone with inside information at Massive Dynamic. Anyone who might possibly know anything is questioned — except for Peter. Turns out that the weapon had an effect on Peter, and it’s weaponized him. He’s hunting down the shapeshifters because he’s tired of reacting instead of acting, and he needs to know what they know. Walter discovers what Peter’s been up to, but he keeps the secret from Olivia, who is just getting over what happened between Peter and Olivia. This could kill her. Literally!