Three years ago, Mark checks into the hospital with a bullet wound in his shoulder. Olivia is concerned and admits that she's not sure she can handle the stress, and that it's different now that they have a daughter. She insists something has to change, and Mark tells her that Stan offered him a surveillance position on a counter-terrorism task force. Olivia warns that he'll hate it, but Mark says it's worth it so that he's there for his family. He promises her that their family will always come first.
Now, Mark moves out to live with Stan and his wife since Olivia won't stand for him choosing the Mosaic investigation over his family. When Charlie sits in the car, Mark gets in with her and explains that he's going away for a little while. She gives him his friendship bracelet back and insists on putting it on. Mark hugs her.
Mark, Stan, and Demetri gather in Stan's office to watch the Dyson Frost tape made during the Somali blackout in 1991, where Frost addresses Demetri ten years in the future. Frost prophetically knows everything that will happen, including the facts that they successfully engineered a global Blackout and almost captured him in Pigeon, UT. He explains that he's had hundreds of Flashforwards, and in almost all of them he dies on March 15, just like Demetri. Now, the next move is Mark's. They speculate as to Frost's motives and Mark finally tells Demetri that they believe there'll be another Blackout. Stan tells Mark to make the next move and stop the coming Blackout.
Simon and Lloyd examine the circuits from the Somali tower, and he confirms they are taken from sketches he did when he was 13. Among the papers are references to tachyonic dark matter, which wasn't known to exist in 1991. Simon concludes that the experiment failed in 1991 because the conspirators didn't have Lloyd and Simon. He insists that this evidence clears them since Frost stole their research in 1991, but Lloyd points out that they know their experiment amplified the effects of the Blackout. Lloyd finds the formula that he saw on the mirror in his flashforward and explains that it's inversion potentially capable of protecting against the Blackout. They need the full equation from the mirror and Simon suggests that Lloyd go see his "girlfriend."
Demetri checks facial-recognition software against surveillance video but has no luck locating Frost, who seems to have disappeared since he faked his death in 1990. Mark checks chess club files and discovers that Frost used a Queen Sacrifice maneuver to beat the reigning champ, Ian McKinnon, who lives in San Francisco. McKinnon has refused Mark's calls and Demetri wants to talk to him. However, first Mark goes over Frost's chess games in 1990. The moves follow a 2-second/4-second pattern and the agents realize it's a phone number in Morse code. The number is for a prepaid cell phone and Frost's recorded message greets them. Mark leaves a message telling Frost that it's his move, and they leave for San Francisco.
Keiko waits at the sushi restaurant from her flashforward where she meets her destined love. Each time someone comes in, she looks up nervously. A waitress suggests that she get a job there, but Keiko insists that she wants a new life for herself.
Nicole comes to see Bryce at the chemo ward where he's receiving treatment. He doesn't want her to see him like that, but Nicole suggests that they go out and enjoy themselves before he becomes ill from the treatment. Bryce agrees but insists on buying dinner.
The SFPD calls Mark and Demetri to an outdoor chess park and are informed that McKinnon has been shot dead. Later, Mark meets privately with Vogel and they conclude that the mole is in their office. Mark comes up with a plan but warns that it'll be ugly.
The next day, Vogel brings in his own team of agents and assembles all of Stan's people in the conference room. When Stan demands an explanation, Vogel says that he's not at liberty to say. Everyone's cell phones are confiscated and they're held in the conference room, along with Simon. They realize that Vogel's people are looking for the mole. They notice that Mark is still free and Stan explains that he's already been vetted. Outside, Mark meets with Vogel in Mark's office, and Vogel starts singing. They find a voice-activated bug designed to resist detection, and his singing set it off. Mark admits that every agent has access to his office, and Vogel says it's time to begin.
In the conference room, Vogel informs everyone that they're looking for the mole, and takes them away one at a time for interrogation. He starts with Demetri, who wonders why Mark doesn't intervene. Mark merely says that it needs to be done. Once they're alone with Demetri, Vogel and Mark explain that they don't suspect Demetri, but they have to make it look good to convince the real mole. Meanwhile, tempers start to flare among Stan and the others. Agent Curdy admits he plays RPGs in his spare time, impressing Simon. Mark arrives and takes Vreede away for interrogation, and then tells them about the micro-transmitter. Simon picks it up and notes that it's the "M" key, and suggests Marcie might be the mole. She ignores him, refuses an offer of sugar for her coffee, and tells Mark that they're all supporting him.
As Keiko walks home, she notices some local neighborhood residents partying. She notices a gang member working on his car and offers to help, and he reluctantly accepts.
Vogel and Mark interrogate Janis, and notice that she's taken a large number of recent personal days. She admits she's been going to a fertility clinic, but the two numbers don't match. Janis finally admits that she's been investigating the mole on her own, and discovered that Marcie has a sick half-brother she didn't declare on her application. He has expensive treatments and Marcie recently received an anonymous $50,000 deposit in her account. As she leaves, Mark assures her that she's doing the right thing but Janis isn't convinced.
Olivia takes Charlie to an amusement park but her daughter would rather have Mark present. Her mother assures her that they both love her and will always be together as a family, and Charlie gives in and asks to go on a ride. They ride on the Ferris wheel, unaware that a man is following them.
Demetri checks Janis' story and confirms that Marcie has a half-brother, but she did declare him on her application. However, the records were divided and it's possible Janis missed the correct application. Mark grows increasingly edgy and finally admits that he moved out of the house because of what he saw in his flashforward. Demetri offers to help but Mark tells him to keep digging on Marcie's half-brother.
Olivia finally notices the man watching them and tries to get away with Charlie. The man approaches them and introduces himself as Agent Danforth Crowley. Mark assigned him as a security detail.
Keiko's new friend Emil takes her to his garage and offers her a job working on ECU engines. When one of the mechanics teases her, Emil tells all of the workers that Keiko is part of the family and they have to treat her with respect. No one dares disagree and Emil tells Keiko to get to work.
Olivia calls and leaves a message for Mark thanking her for the security detail. Lloyd comes in and says that they need to talk. In her office, he asks Olivia what she remembers and explains that the mirror formula relates to the global blackout. She remembers seeing it written on the mirror in lipstick and writes down what she can. It's the formula for halothane, an anesthetic, and Lloyd realizes that they were working on QED: a quantum entanglement device. When Olivia leaves on her rounds, Lloyd apologizes for bothering her but Olivia says she understands.
Keiko takes Emil to the sushi restaurant and explains why she visits there so often. She explains that it was the first day where she was happy as she followed her heart. Keiko asks Emil what he saw in his flashforward, but all he'll say is that it was something different than her. She thanks him for giving her the job and he says that he's happy to have her.
As they walk through Little Tokyo, Bryce describes his flashforward to Nicole. As they arrive at a sushi restaurant and examine the outside menu, he suddenly leans forward and kisses her, and then apologizes. Nicole half-heartedly suggests that he was thinking of Keiko, Bryce accepts her explanation and they go for a burger... unaware that Keiko is inside.
The next day, Keiko and Emil are working at the garage when ICE breaks in on an immigration raid. The police arrest Emil for employing undocumented workers. When they determine that Keiko doesn't have papers, they arrest her and many of the other workers and haul them off.
As Bryce drives to work he calls and leaves a message for Nicole, apologizing again for the kiss and thanking her for the great time. He looks up as the ICE van drives by him.
As Stan's team is kept confined for their 36th hour, Demetri goes through Marcie's background files. As they check surveillance footage of her, Mark notices that she often stands outside and pours sugar into her coffee. He remembers that earlier she said that she doesn't take sugar in her coffee, and realizes that she's sending a signal every time they made a move in the Mosaic investigation. They look at Marcie in the conference room and go for their guns. She grabs another agent's gun, opens fire, and shoots her way out. Outside, a motorcyclist picks her up and they try to make their escape. Janis gets outside and shoots the driver, and then captures Marcie. Mark notes to Janis that she was right, but she wishes she wasn't.
Later, Stan talks to Mark, who explains that he was under orders not to explain to him what was going on. Stan figures he was manipulated for the greater good in a Queen Sacrifice, and just smiles and says he has his sources when Mark knows what he and Demetri discussed earlier. After Stan leaves, Mark receives a call from Dyson Frost. Frost congratulates him on finding the mole and promises that he'll be in touch for a personal meeting now that he's sure Mark can keep his leads alive.
That night, Simon and Janis leave the building. As they go, Janis notes that the x-rays suggest that Simon killed his Uncle Teddy. She observes that he did it to send a message to Flosso's superiors... and she's received the message. Simon realizes that she's a double-plant, in case Marcie was caught. Now no one will suspect that she's the other mole.