I liked this episode, although in retrospect it was a little thin and didn't really present us with any huge forward momentum. This fifth season has been a year of incremental nudges in terms of narrative, but "Through the Looking Glass and What Walter Found There" was at least entertaining and suitably intriguing for long stretches. I just wish the end result felt more worthwhile, without instead leaving us with so many unanswered questions. For that reason, I can't really say this episode left me rapt, but there was some interesting stuff to consider.
The season-long scavenger hunt for materials required to implement Walter's (John Noble) plan to rid the world of The Observers was once again the backbone of this episode, although the difference here was seeing Walter take it upon himself to excavate the next video-tape and follow its peculiar instructions—taking him to a half-destroyed location downtown where he could access a "pocket universe" to retrieve something or someone he's kept hidden there. Naturally, Peter (Joshua Jackson), Olivia (Anna Torv) and Astrid (Jasika Nicole) weren't too far behind once they realised Walter's uncharacteristically gone walkabout by himself.
A great deal of this episode was simply keeping viewers on the hook with the possibility of answers, but very few actually came. We learned that Walter used the pocket universe to hide the Observer-like boy last seen in season 1's "Inner Child" (whose abilities as an empathy clearly have use), but nothing more was really forthcoming. It was more a chance for director Jon Cassar to have some fun with Walter in the Fringe universe's version of Wonderland—where things were topsy-turvy and time itself was running too quickly (five days in the pocket universe equaled two decades in the outside world). Later, an old radio tuned to a single frequency was discovered. I'm sure all of these things will come in useful very soon, but it's probably not worth theorising about just yet.
I suppose my big problem with this episode is how it was a shameless way to just give the characters some items, and despite some good sequences (including a beautiful moment when Walter first rediscovered the portal to the other universe) I found it hard to really get invested in much of it. The show's kind of going through the motions when the gang are essentially just gathering trinkets to be used in the season finale. More engaging was seeing Peter console himself by playing his dead daughter's hologram answer-phone message on a loop, before Olivia arrived and implored him not to exclude her from his grieving process; or the sheer fun of Peter trying out his new super-powers against an Observer towards the end, which was going for a Neo vs Agent Smith vibe in the confrontation. I also liked the final scene, where Walter made Peter promise he'll help keep his humanity intact (after feeling guilty about his callousness over Cecil, a man trapped in the pocket universe he treated as "collateral damage" throughout the whole episode until his demise), unaware that it's Peter who needs help in that department. The implant he's injected himself with to imbue himself with Observer-like powers appears to be having a negative effect on his own compassion, and he's already seeing the world as an artificial blue haze like the Observers. (Another nod to the Agents in The Matrix, who only see things in green streams of code.)
Overall, "Through the Looking Glass and What Walter Found There" was one of those episodes that basically existed to impart some information that will be more critical in the future, but it did so in a largely entertaining way. It helped that there were three solid character moments, too, but otherwise I don't imagine many people will be keen to ever watch this episode again. It's a necessary part of a bigger story being told, not a very good story in its own right.