So Walter follows the clues and meets the keyboardist for his old favorite band, Violet Sedan Chair (check out this clue about them from, like, two years ago), and finds out that the band broke up because of the mistake-chain. Because whenever Walter gets to save something he loves, he loses something else. But the upshot is that Peter wasn't killed and there's an opening for Christopher Lloyd to come back for other guest spots!
I was concerned when Fringe was moved to the Friday Death Slot, but it looks like the move hasn't damaged the show at all. This is as complex and clean an episode as Fringe ever made. Even though this could have been a simple throw-away episode, a space-filler to bring the show back without changing the status quo too much, but they went ahead and produced this beautiful clockwork of an episode instead. Every little thing that happens in the whole thing fits into everything else that happens, and it all dovetails nicely with what we already know of Peter and Walter's shared history. It's brilliant.
More thoughts on Fringe, "Firefly"