At the Milton-Staller wedding in Brookline, Massachusetts, the friends and family arrive to celebrate with David and Shelley. Mrs. Staller goes to make sure that Shelley is doing well and welcome her to family. David and the groomsmen are having a drink. The grandmother, Nana, notices a young blonde man standing in the door and asks her friend Lynn who he is. Lynn doesn't know and Nana watches him suspiciously.
David has trouble breathing and uses his asthma inhaler. Meanwhile, Nana insists that the man she's seen can't be who she thinks it is. She gets up and points at him, yelling at him. She starts to choke and many of the wedding party guests start choking as well, as the blonde man turns and leaves.
Detective Burt Manning is on the scene when Olivia arrives. He tells her that there are 14 victims so far. Walter and Peter arrive behind her, Walter awkwardly driving while talking about his wedding. He wants Peter to wear his tuxedo on his wedding day, and wonders if Agent Dunham will call him Dad. He thinks that they'd be perfect together, but Peter isn't convinced. Inside, Walter examines the bodies and determines that they died of asphyxiation despite the fact that their airways are clear and there's no fluid in the lungs. Olivia determines that all the victims were on the groom's side of the family, and they suspect that they were all exposed to a toxin. She notices that one of the victims has a tattoo on her arm and realizes that she's a Holocaust survivor, and wonders if it was a deliberate attack. Peter and Olivia check the room where the groomsmen occupied and hear a thumping noise from a closet. David staggers out, gasping for breath and dropping his inhaler before dying.
At the lab, Walter and Astrid confirm that all the victims are the descendants of Eva Staller. The survivors are not relations, including one illegitimate grandchild. Walter cuts open the chest of one victim and discover that the blood is blue due to a lack of oxygen. He concludes that the victims suffocated from the inside out. Walter takes a breath from the respirator and realizes it kept David alive longer than the others because it weakened the toxin.
Olivia talks to Shelley's mother and asks if she saw anyone who wasn't invited to the family. Mrs. Milton describes how Nana pointed out the man and Olivia asks her to examine the wedding video and see if she can identify the man. Peter examines the candle and finds one candle that is cinnamon rather than jasmine like the others. They take it to the lab and Walter runs the wax through the spectrometer. Shelley's mother has identified the man on the tape and Astrid runs the face through the FBI database. The spectrometer reveals that the candle contains the toxin, a derivative of hydrogen cyanide. Olivia wonders why it only killed some of the guests and Walter remembers how during World War II, Josef Mengele wanted to create a weapon that could target certain individuals and ignore others. Walter speculates that the wedding was an experiment to test the validity of the weapon. He figures that the persons involved will test the success of their weapon by trying again.
At a cafe, the blonde man orders hot tea at the counter. He then sits down at a table where a woman is talking to her daughter Jordan. The blonde man sits down and compliments the girl, and then takes out a vial with a dark liquid, shakes it up, and pours it into his glass of hot water. The mother notes that it smells like cinnamon, and the tea starts to bubble.
Later, the Fringe team arrives at the café where nine people suffocated to death. There are no surveillance cameras to have captured anything, and none of the victims are related. Olivia notices the cup of hot tea that smells like cinnamon. Broyles has it dusted for prints while Walter realizes that all of the victims had the same brown eye color. He concludes that the toxin targeted the common genetic trait, and notes that Broyles was lucky not to be there. Walter has an idea and insists on driving back to the lab, but Peter takes the keys. Across the street, the blonde man sees Walter and asks a police officer if he's Dr. Bischoff, and comments that Walter looks just like his father.
At the lab, Olivia explains that they could only pull a partial print from the tea cup and couldn't get an ID. Walter shows them a molecular display of the toxin. It can be programmed to target any group with similar physical or genetic similarities. Peter notes that one of the ingredients, chromium trioxide, is highly regulated and Astrid starts searching for sources. Walter notes that many chemists leave a signature and they spot an image of a sea horse. He realizes who created the toxin and goes to the office. He remembers that his father, Robert Bishop, was called The Sea Horse was because he was a skilled swimmer. He lived in Berlin in 1943 and worked as a spy for the Americans. Walter starts going through his boxes of files while saying he saw the formula in one of his father's German novels. Peter finally tells his father that he sold the books 10 years ago because he needed money. An angry Walter blames Peter, insisting that someone got the formula from one of the books and now Robert's work is killing people.
The blonde man goes to Peter and Walter's house and takes an apple from the refrigerator.
Peter and Olivia call Broyles and tell him what they discover. They go to the bookstore where Peter sold the book to try and trace the seller. The store owner, Markam, claims he doesn't remember who bought it but Peter points out that he keeps a log. When he goes to check his log, Olivia points out that there are plenty of valuable things that Peter could have sold for money. He admits that he sold the books out of anger at Walter, and he's not proud of it. Markham gives them the address of the buyer, Eric Franko.
The blonde man works in a secret lab, preparing more of the toxin.
When they realize no one is home, Olivia and Peter break into Franko's home and find a collection of Nazi memorabilia. Franko comes in and they hold him at gunpoint, and the artist explains that he's an artist doing a work condemning tyrants. He shows them that he took apart the pages and used them as collages. Peter wants the books back and Franko reluctantly agrees to donate his works.
Back at the lab, Walter stares in anger at the collage of Hitler and refuses to accept Peter's apology. Since Franko's work was never displayed, they realize that the killer couldn't have gotten the formula from the books. Walter has analyzed the killer's DNA, taken from the oil in the partial fingerprints, but Astrid is unable to get a match. An analysis of the oil from the fingerprints reveals that the owner is apparently a hundred years old. Peter has an idea and asks if the toxin could target a combination of traits, and Walter confirms that it can.
The blonde man goes to a tent where a homeless man is living. He invites the man to watch, and then lights a sterno can with the toxin. He steps back and the toxin rapidly spreads out, killing the man.
Walter is experimenting on rats to recreate the toxin, while Olivia comes up with the names of three suspects who have bought the necessary chemicals. Two of them are companies and the third is an home address, and Olivia calls in a team to meet them there.
The blonde man is creating a fake ID at his home.
Olivia, Peter, and Walter arrive as an FBI team wearing gas masks enter the house. The blonde man hears them above. He starts packing away sterno cans containing the toxin and then turns on a burner with a beaker of the toxin on top of it. As he slips out, Olivia finds a hidden door and goes downstairs. They find discarded papers and vials labeled with various genetic traits. Walter explains that the killer plans to create the Master Race by process of elimination. Olivia finds the discarded photos from the killer's creation of a fake ID, while Peter discovers Walter's sweater. Walter starts to choke and Olivia sees the beaker and realizes what's happening. They get Walter outside and provide oxygen just in time.
The blonde man gets in line with others and waits.
Walter recovers and asks for his sweater back. Olivia wonders why the killer targeted him, and Walter figures that the killer knows that Walter's father was a spy. Olivia finds the discarded photos of himself the killer used to create his ID, and a piece of glass with a logo on it, a logo that Peter recognizes.
Security stops the blonde man and check the box of sterno cans. They find nothing and he goes inside to the conference of the World Tolerance Initiative.
Olivia and Peter drive to the conference center and call to tell Broyles what's going on. They have no idea who the killer may be targeting and warn Broyles that they're running out of time.
Walter goes down to the basement and carefully removes his gas mask. He spots a piece of equipment, recognizes it, and takes it with him.
At the conference, the blonde man disguises himself as a caterer and places his sterno cans beneath the heating trays.
Astrid comes to get Walter, who is working with the killer's chemicals. He tells her that he can stop the murders.
Peter, Olivia and the FBI agents spread out to search for any candle or heat source that could set off the toxin. Walter and Astrid arrive and Walter insists on going to the higher floors. Olivia moves through the crowd looking for the killer. While Peter checks the bar. He spots the heating trays and stops a caterer from lighting one. Peter stops her, but they hear a man coughing. They run to the spot and find… the killer. He's asphyxiating and looks up, where Walter is using the equipment to distribute the toxin. The blonde man yells "Traitor!" and then dies, while Walter walks away.
Afterward, Walter explains to Broyles and Olivia that he used a sample of the killer's DNA to create a version of the toxin that would only be fatal to him. Walter tells Broyles to do what he has to, but he doesn't regret what he did. Broyles considers him for a moment, and then tells him good night and walks away. Walter explains to Olivia that the killer corrupted Robert Bishop's work, and that there's nothing he wouldn't do for family. Olivia turns and leaves without a word.
Later, Peter returns home and gives his father a present: all of Robert's work that Franko left intact. Walter finds a photo of Robert and shows it to Peter, and notes that Peter and his grandfather were much alike. Peter wonders how the killer found the toxin, and Walter notes that there are some mysteries destined to remain unsolved. He thanks Pete for finding the books and sets aside some photos… including one of the blonde man, unaged, behind Robert Bishop.