At the Milton/Staller wedding in Brookline, Mass., the groom's side of the family is wiped out when they all start choking. The cause of death appears to be asphyxiation, even though there was plenty of air in the house. The groom bites it too, but after everyone else, because his inhaler was keeping him alive. The alone clue is the groom's grandmother spotting a man she recognized but no one else could identify. The concentration camp ID number on her arm -- and the fact that all the dead are her descendents -- leads the Fringe team to speculate it was a targeted attack.

Peter, keen nose that he has, notices a cinnamon candle in among all the jasmine candles chosen for the wedding, which is too ridiculous to think about too much. Walter starts to figure it was much scientific experiment as targeted attack, a theory which gains traction when the evil guy attempts to replicate his results in a coffee shop. In this case, however, the victims weren't all related, but Walter does figure out the common bond: brown eyes. The attacks are targeted genetically. While the Fringe team investigates, the evil guy spots Walter Bishop: "He looks just like his father," he says.

While investigating the toxin, Walter discovers a unique chemical signature: a seahorse, which leads Walter back to the work of his dad: Robert Bishop. Er, Robert Bishoff, a scientist who worked with the Nazis Don't worry, though! Bishoff was actually spying for the Allies. Whoever's killing people is carrying on the work that was done back then, which Walter thought was safely tucked away in his father's old books. Unfortunately, Peter sold those both for money and to get back at an absent father, and so the two of them struggle with the guilt that they loosed this evil knowledge on the world. (Well, Walter not so much: he blames Peter.)

As the FBI closes in on evil dude Alfred Hoffman, he sets a trap for Walter -- and almost succeeds in killing him -- before heading to the "World Tolerance Initiative" to try to wipe out a bunch of non-Aryan peoples by putting his toxin in those little candle things restaurants use to keep serving dishes warm. Peter figures it out while Walter has whipped up a batch of Hoffman-specific toxin, which Broyles is a little bit cross about. But Walter stands up to him, which isn't the only bit of odd similarity with the last couple of episodes: it's an episode in which people fall prey to toxin, and Walter cryptically alludes to whatever happened to Peter all those years ago. Meanwhile, it's never explained how Hoffman is apparently as fresh as those halcyon days of the Third Reich. There may be more answers coming.

Also, Peter tracks down his grandfather's books with the help of his friend Markham. That's good! A conceptual artist has incorporated the German science into artworks as part of a "banality of evil" series. That's bad! But it does mean that Walter's father's work wasn't responsible for the current attacks. That's good!

Next week: the winter finale. That's good! And also bad!