So that's it? Game over, man? Not quite. The Walking Dead TV show is unique in that you can survive a zombie bite, but you have to amputate the infected limb immediately. So the rest of the episode addresses the tension of Tyreese waiting for Michonne to arrive, chop off his arm, and (hopefully) save his life. Meanwhile, Tyreese's fears take shape around him as fever visions of dead characters, each having lots to say about his current state.
The Walking Dead tried something like this in Season 3, when Rick got a "phone call" from his dead wife. It was a strange sequence that only existed because the comic book did it fairly well... before running it into the ground, anyway. The fever visions, however, are much better by comparison and bring back several deceased characters for one last hurrah. The Walking Dead is only going to get to do this once after all, and thankfully gets it perfect the first time.
We have friendly characters, like Bob and Beth. We have hostile enemies, like that cannibal and the Governor. Others, like Lizzie and Mika, are downright creepy considering why they aren't in the show anymore. But all of them reflect Tyreese's internal struggle very well, questioning the futility of his actions even as he fights to live. There are some genuine twists and turns leaving you never quite sure how the scene will end, all coming together for a surprisingly powerful conclusion.
Meanwhile, Michonne steps up to become the new voice of unexpected optimism for the survivors. It's a little strange, coming from her, but she manages to pull it off even better than Tyreese did mere moments before. While Rick and Glenn wallow in their moral dilemmas, she pushes them into considering whether they really want to live in a world without hope, scavenging for supplies forever. Tyreese's current predicament aside, that's a welcome perspective for The Walking Dead - the show can't just be about horrible things happening all the time, or it loses context and is far more painful to watch. Besides, all the terrible events of the past two seasons technically occurred in the span of maybe three months, a pretty short timeframe to assume hope is lost forever.
I suspect the thrust of Season 5 will follow the comics - where Rick and the group decide once and for all between hope and nihilistic survival. But that decision has no context without episodes like this - where hope is smashed to pieces against cold, hard reality - and "What Happened" pulls it off very well. And that's without mentioning the episode's many technical achievements, like wonderful makeup effects, effective foreshadowing, or treating zombie kills as an art form. Regardless, this is a powerful return to The Walking Dead and continues to prove this season is the best yet.
Bottom Line: The Walking Dead welcomes you back with open arms, just before tearing out your heart and eating it in front of you. But you'll be riveted the entire time, and will want to return next week for more.
Recommendation: Run, don't walk, to anywhere you can catch up on the latest episode.