Two women notice an unfamiliar man standing in the back of the room.  Shortly after, an elderly woman yells “It’s him!” and points to this man.  She falls to the ground, unable to breathe and dies.

Agent Dunham arrives at the wedding site and is told there are fourteen victims so far.  Walter begins examining the victims.  They have all died of asphyxiation in a room full of air, and all were on groom’s side of the family.  Dunham notices the elderly woman had a tattoo and was a holocaust survivor. 

The groom suddenly flies out of a closet gasping for air, falls to the ground and dies.  What a freaky scene!

Back at the Harvard lab, Walter and Astrid discover that all fifteen victims were direct blood relatives of Eva Staler, aka the first victim. Walter begins to dissect the groom’s corpse, and notices he has blue blood.  Something must have bonded with the victims’ hemoglobin, changing the blood’s color and robbing its body of oxygen.  But how was the toxin delivered?

Agent Dunham and Peter begin interviewing witnesses.  A woman tells them about the unknown man standing at the back of the room.  She also casually mentions that all the candles are jasmine, but Peter notices they smell like cinnamon.  This was how the toxin, hydrogen cyanide, was released into the air.   If the toxin was airborne, then why it did only kill some of the guests?

The Nazi’s were pioneers in creating weapon that could target specific people.  Walter believes the wedding murders were a scientific experiment.

Next you see the evil Nazi killer in a coffee house, ordering tea.  He pours a black liquid into his steaming cup. Vapors begin to spread around the room.

The Fringe team arrives at the coffeehouse; there were nine victims. This time the victims were not related to each other.  Agent Dunham finds a cup of tea that smells like cinnamon.  All the victims had brown eyes, which is a common genetic trait.  The killer has figured out how to program the toxin to kill any particular genetic group.

While researching the genetic makeup of the poison, he notices what looks like a seahorse, and knows the formula was created by his father.  Walter remembers his father hid the formula from the Nazis in books, to take them out of Germany.  While looking for these books, Peter admits he sold them for money. This must be how the Nazi killer got a hold of the formula.

At the used bookstore, Peter finds out Eric Fineco bought the books, but he turns out to be a lonely artist.  Apparently the killer didn't recreate the formula from the books.  There isn’t a match on the DNA from the killer’s skin cells either, which said the man was over 100 years old.

The Fringe team locates a buyer of sodium chromate, the sister compound needed to create the toxin and arrives at the Nazi killer’s residence. 

While searching the basement, Dunham finds a small picture in the garbage.  He must have been trying to make an ID, but to what?  Peter also locates Walter’s sweater in the basement just as Walter begins to have trouble breathing.  They get him out just in time!

Dunham discovers the logo of the World Tolerance Initiative in the basement and calls Agent Broyles to evacuate the conference.  He is trying to kill the world leaders.

After mixing a substance, Walter asks Astrid to take him to conference.  He is determined to stop the killer.

Peter finds the toxin just in time, but then hears someone coughing.  Peter and Agent Dunham race over and see the Nazi killer gasping for air.  Before he dies he yells out, “Bishop, traitor” and points to Walter on second floor. 

Peter brings back all Walter's father’s books.  Walter shows him pictures of his grandfather, saying there is a family resemblance between the two.  The episode ends focusing on a black and white picture of the Nazi killer and Walter in a lab.