Ben leaps into a man in a trenchcoat late at night. A man named Lawrence stumbles out of the shadows, bleeding and pleading with “Henry” to find something before the others can find it. He whispers the word, “Pathfinder,” before dying.

At HQ, Jenn is looking for Magic. She’s told he’s taking a personal day. Tom says they have to decide who is going to guide Ben. Jenn and Ian play rock-paper-scissors and Jenn wins. Ziggy finds Ben but then there’s an activation error that Ian can’t explain.

Addison starts to panic, but Ian reassures her they haven’t lost Ben. It’s an internal system error. Just then, Ian’s phone buzzes and they receive a message from an unknown number, “Do I have your attention?”

Ian excuses themself to check the backup servers. Jenn is suspicious. In the backroom, Ian calls the number. The contact is a woman that works for Rachel’s boss. She demands that Ian continue to share information gathered by using the quantum chip her employer provided.

Because Ian and Jenn locked the employer out, they are unable to reactivate the chip every twenty-four hours as required.

She extorts an agreement from Ian to share the data in exchange for one more day of activation.

Ben is being interrogated by police about Lawrence’s murder. As Professor McCoy, Ben reports Lawrence’s last words were that the killer had cold, blue eyes. The police are skeptical and question whether Ben has Communist ties. Jenn arrives suddenly.

She discovers McCoy was a sergeant in WW2. Ben puts the police in their place but they tell him not to leave town.

Jenn fills him in that it’s 1955 and they’re in Princeton, New Jersey. Ben is a visiting physics professor and a suspect in an unsolved murder.

Ben is ecstatic to learn where and when they are because Einstein was working at Princeton that year. Einstein is the reason Ben fell in love with physics in the first place.

Unfortunately, Einstein died a month previously. The murdered professor had written letters to Ben’s host asking for help finding a missing formula created by Einstein.

As Ben’s refuting the possibility of a lost Einstein formula, they are interrupted by several men in suits from the Atomic Energy Commission. They ask Ben to go with them.

When he asks if he’s under arrest, Hannah steps out from behind the suits to ask for his help.

They follow Hannah to a secure building. Ben recognizes the work as Project Matterhorn, a controlled thermonuclear research program that led to the first hydrogen bomb.

Dr. Donovan, Hannah’s project supervisor, introduces himself to Ben. He invites Ben to sit and dismisses Hannah.

Donovan explains that Einstein had sketched out a formula that solved all the problems with nuclear fusion days before he died. Lawrence had found clues to its whereabouts but was killed before he could find it. He never shared the clues with his colleagues, wanting to wait for McCoy to arrive.

Donovan warns that Soviet spies are watching them and looking for the formula too.

Ben commits to finding the formula.

He finds Hannah schooling some of the Matterhorn NAS fellows on a formula equation. She offers to be his guide around campus. She mentions she’s working on a paper that Donovan will help her get published. Ben addresses her by her first name and has to cover with a story about Dr. Lawrence mentioning her in his letters.

Jenn arrives and reveals that Hannah never gets credit for her papers. Donovan publishes them under his own name. Ben is enraged by the unfairness of that. He calls Hannah back and requests that she help him with the search for Einstein’s formula. He promises to share credit for the discovery with her fifty-fifty.

Hannah asks if they’ve ever met before and Ben has to say no. They decide to search Lawrence’s apartment for the clue to Einstein’s formula.

At HQ, Jenn wonders at what the repercussions will be if Ben discovers the secret to fusion in 1955. Tom wants them to figure out who else is looking for the formula. Addison starts looking into information on Russian spies at Princeton.

She asks Tom if it’s weird seeing Ben walking around his alma mater. Tom points out he didn’t go until fifty years later. He redirects her attention to Ben and leaves.

Hannah and Ben find Lawrence’s rooms ransacked. Ben finds a picture of McCoy and Lawrence together from their military days. Jenn arrives and tells Ben the 8th Airborne Division had a codename. Ben remembers Lawrence whispering, “Pathfinder.” He opens the back of the picture frame and finds a drawing with the word “einsamkeit” (German for solitude) written below it.

Hannah recognizes Einstein’s handwriting and the window he drew.

She says the window is in the physics library. Ben’s excitement is negated by Jenn’s horror as them finding the clue has altered the timeline and now Hannah dies that night.

At the library, Hannah tries to talk the librarian into giving them access while Jenn fills Ben in on the experiment at Project Matterhorn that destroys the building and kills Hannah.

Ben shares that Hannah seems to know that he’s him.

Jenn gets a text on her cell phone and says she needs to step out. She tells Ben to find the formula and keep Hannah away from the Project Matterhorn building.

Hannah comes to get Ben and they begin their search.

At HQ, Addison asks Jenn if everything’s okay. Jenn says there’s a security thing and throws the handset to Addison.

Jenn finds Ian who is freaking out. She reminds them they don’t need to fix this on their own. She reminds them who they need to talk to.

Ben and Hannah go through every book in the library and find nothing. Watching them from HQ, Tom tells Addison they need to search the second floor. Addison reminds him she can’t go back in because Ben doesn’t want her there. Tom realizes that he’ll have to go in.

Ben’s unimpressed with Tom’s arrival but Tom explains that he found a secret room on the second floor when he was a student at Princeton.

Tom reveals to Ben that he found the room while getting high as a college student with his future wife. Jenn returns and is shocked to see Tom in the leap.

Tom explains his wife died of cancer six years ago and redirects Ben to the leap’s mission.

Hannah calls Ben over and shows him that Einstein’s sketch doesn’t just match the window, it matches the face of a grandfather clock she’s discovered built into the wall on the second floor of the library.

Tom walks through the wall and reports the room is on the other side.

Ben notes the clock is running three minutes fast. Since Einstein stated that time is relative, he moves the clock’s minute hand back three minutes and the door opens.

In the room, they find one of Einstein’s notebooks. As they’re looking at it, lit only by a kerosene lamp, a man comes up the stairs. Tom alerts Ben to the blue-eyed man’s approach. Ben tries to shut the clock door but the assailant forces his way in, causing the lamp to be knocked over, lighting the room on fire.

Hannah manages to knock the assailant out with a heavy bust. They drag the man out, Tom reminding Ben to grab the journal. Outside, Ben tells Hannah to call the operator for police and fire.

The ID on the assailant is for Klaus Erikson. Tom doesn’t think Russians are the problem.

Tom returns to HQ. The timeline now shows Erikson convicted in the murder of Professor Lawrence.

Tom has Jenn pull up information on Erikson and Project Paperclip. Addison informs Jenn that after WW2, the US recruited ex-Nazi scientists. Thousands of them.

Addison checks in on Tom, apologizing because she’d forgotten he and Kate, his late wife, had met at Princeton, and she hadn’t considered that when she asked him to go into the leap.

Hannah and Ben have a drink together while Ben translates Einstein’s journal from German. They toast to “Al.” Ben finds a page written in Greek. Hannah looks it over. She realizes Einstein used ancient Greek as code to hide the formula. She begins translating it. Tom arrives and tells Ben Hannah needs to stop. Ziggy’s already decrypted the formula and it’s not a fusion formula, it’s a bomb.

Ben tries to convince Hannah she needs to stop but she grabs the journal and is intent on completing the decryption. In a desperate attempt to keep her from leaving, Ben shouts out, “Agent Robert Cook” the name of the host he was in when he first met Hannah.

He describes their adventure together in New Mexico. When Hannah asks how he knows the details, he admits that he’s a time traveler.

Tom reminds Ben he’s breaking protocol. Ben tells Tom to leave him alone. Tom returns to HQ. He explains to Hannah who he’s talking to and how he leaps through time. She’s amazed and delighted. She reminds him that she recognized him despite him being in a different body.

Ben explains he thinks he’s there to help Hannah. She figures out that Donovan’s never going to publish her work. Ben wants her to sit tight while he talks to Donovan. He tears the Greek page from the journal and hands it to her.

He leaves and she sets the page on fire.

Ian finds Rachel in the parkade. They ask her to rewrite the firmware to jailbreak the quantum chip. Rachel knows that her employer will fire her and destroy her career but agrees to do it.

At HQ, Jenn’s combing through the Paperclip files for the identities of the Nazi scientists recruited by the US. Many of them have fencing scars they wear as badges of honor. Tom and Addison recall Donovan had fencing sabers on his wall. Jenn discovers Donovan is also an ex-Nazi scientist. Tom heads into the imaging chamber.

Ben’s showing Donovan the journal without the formula, pointing out it is a major scientific find and Hannah should get full recognition for the discovery. Donovan is frantic for the formula.

Ben admits he saw the formula. Donovan asks if he could recreate it. Tom arrives to inform Ben that Donovan’s a Nazi and behind Lawrence’s murder and the attempt on Ben and Hannah’s lives in the library.

Donovan reveals that he’s had a henchman kidnap Hannah and bring her to the lab. He pulls a gun, points it at Ben, and promises to kill him unless Hannah recreates the formula from memory.

Tom gives Ben some pointers on how to disarm Donovan. Ben’s able to redirect the gun but Donovan grabs one of his sabers. Ben is forced to defend. Meanwhile, Hannah’s fighting off her goon. She gets herself free just as Donovan is about to skewer Ben. She turns on the stellarator and the saber flies out of his hand, attracted by its magnets.

Ben punches him out.

The police arrest Donovan. Ben and Tom step away. Tom informs Ben that Hannah gets recognition for her work. Donovan and his Aryan gang are arrested. McCoy gets a medal for this adventure.

Tom falls silent looking at a young couple carving their initials in a nearby tree. He tells Ben that he and Kate would sit under that tree as students and look at those exact initials.

Ben and Tom pay respect to each other’s loss and part ways.

At HQ, Tom and Addison hug. Tom admits he misses Kate today. Addison understands.

In a dark parking lot, Ian gets into a car with the corporate lackey, Simone. They refuse to give into her demands. Jenn knocks on the window. Ian introduces them. Ian points out that Simone blackmailed a federal employee that morning and their conversation is now being recorded.

Simone retaliates, promising the quantum chip will remain offline forever. Jenn says they’ll take their chances and Ian gets out of the car, leaving a bewildered Simone behind.

Hannah and Ben take a walk. They know he’s going to leap soon. She wishes they had more time but begs him to answer one question for her. He agrees and she asks for his real name.

He tells her and she is confident that they’ll meet again. As she begins to explain her scientific theory on how they’re being drawn together, he kisses her.

As they’re kissing, he leaps.