The Season Finale.
The Lifeboat returns to the present, where Lucy's fiance Noah patches Rufus up. Flynn jumps to Washington, D.C., in 1954 to put an end to Rittenhouse by blowing up the site of their summit. The team jumps after him, but Jiya is forced to join, as Rufus is too weak to pilot the craft, despite the uncertainty of putting four people into a three-person time machine. Agent Christopher is arrested, only to be freed by Mason, who reveals to have been secretly working against Rittenhouse all this time. In the past, Flynn blackmails Joseph McCarthy for the location of the summit and has him arrest Lucy and Wyatt as Soviet spies. They escape and trail Lucy's grandfather Ethan Cahill to a gay club, where they convince him to help them stop Rittenhouse. At the summit, Lucy convinces Flynn to try a nonlethal way of stopping the conspiracy. Jiya suffers a seizure as a side effect of time travel, so Rufus and Wyatt jump back to get her to a modern hospital. Lucy stays with Flynn and convinces Ethan to help them by keeping detailed records of Rittenhouse activities and members over the next several decades. In the present, Lucy meets with the aging Ethan, who gives her all the records, which Christopher and Mason use to start arresting Rittenhouse members, starting with Neville and Lucy's father. Rufus and Jiya admit their love for one another, only for Jiya to have a Flash Forward to a possible future (or, perhaps, a Flash Sideways to an alternate present), where the Golden Gate Bridge is in ruins. Lucy gives Flynn the name of the Rittenhouse member, who ordered his family's death, only for Christopher to follow her and arrest him. Christopher gives Lucy a chance to restore her sister with one last trip. Wyatt appears to be finally accepting Jessica's death as inevitable and may harbor feelings for Lucy. Lucy visits her mother and tells her about the time machine and Amy. She is shocked, when her mother reveals to come from a longstanding Rittenhouse family. Emma reclaims the Mothership and is implied to be a Rittenhouse agent.
- Almighty Janitor: For a pediatric surgeon, Benjamin Cahill sure has a lot of pull. Justified, since most Rittenhouse members actually have nothing to do with politics and act behind the scenes as part of a larger network.
- Amoral Attorney: It turns out that a great many modern members of Rittenhouse are corporate lawyers.
- Asshole Victim: When escaping, Wyatt beats the crap out of Joseph McCarthy. Doubt that anyone feels remorse over that.
- Cannot Spit It Out: Rufus admits to the unconscious Jiya his hesitation in telling her how he feels despite facing mortal danger every day. He promises to be more open, if she gets better. Later, she reveals that she heard every word and feels the same.
- Cliffhanger: Lucy finds out that her mother is a part of Rittenhouse, and Emma reclaims the Mothership and might be a Rittenhouse agent as well. Jiya sees a possible future/alternate present involving destruction.
- Corrupt Bureaucrat: As expected, some members of Rittenhouse hold government positions, although not as many as expected.
- Defector from Decadence: Lucy's grandfather Ethan Cahill wants nothing to do with Rittenhouse and stays only out of fear. He readily agrees to gather compromising material across decades in order to give the authorities enough to arrest the entire conspiracy.
- Evil All Along: The ending implies that Emma may be a Rittenhouse agent.
- Lucy is rocked to discover Carol is "Rittenhouse stock" and makes it clear she couldn't care less about having a daughter back, she just wants to keep up their mission.
- Flash Forward/Flash Sideways/Flash Back: As a side effect of being a fourth person in a three-person time machine, Jiya starts experiencing visions of possible futures (or alternate presents), which appears to involve the destruction of the Golden Gate Bridge. Alternatively, she may be seeing the construction of the bridge in The '30s.
- Frame-Up: McCarthy's people casually plant a Soviet gun on Wyatt to arrest him as a spy.
- Good All Along: Mason was only pretending to side with Rittenhouse, tricking them into giving him access to the NSA servers, which he uses to keep tabs on known Rittenhouse members.
- Hope Spot: Mason turns out to be good, between his and Ethan Cahill's efforts, Rittenhouse is starting to be dismantled and Lucy's promise to be able to go back and rewrite her sister back into the present is about to be made good on...and then she goes to talk to her mother and come clean on what has been going on where it's revealed that she, too, is Rittenhouse, has no interest the past being written back, and that they have an operative about to rewrite history in their favour.
- Like a Son to Me: Mason still views Rufus this way and is determined to keep him safe, knowing full well that he himself is already damned for what he has done.
- Not What It Looks Like: When NSA agents are arresting Flynn during his meeting with Lucy, she desperately tries to convince him that she didn't know anything about it. Flynn doesn't believe her.
- Outdated Outfit: Inverted Trope: when they arrive to 1954, Lucy is still wearing her early 1960s outfit from the previous episode when they were headed to 1962 (to erase Flynn from existence). As she and Wyatt turn to go out, Jiya asks Lucy if she's really going to wear that in this decade.
- Screw This, I'm Outta Here!: Flynn's Mooks are finally tired of the never-ending conflict through time and bail, leaving just Flynn and Emma to carry on the fight.
- Secret Legacy: Lucy's mom turns out to be a member of Rittenhouse too. In fact, both the Cahills and the Prestons are longstanding Rittenhouse families, making Lucy "almost royalty".
- Straight Gay: Lucy's grandfather Ethan Cahill may be gay, but he doesn't behave like a stereotypical homosexual, mostly because, in The '50s, being outed as gay was tantamount to suicide, which goes doubly for the more concervative Rittenhouse. This is why he was married and had a son, while still frequenting gay clubs.
- Take a Third Option: Rather than accepting her role as a part of Rittenhouse or allowing Flynn to kill Rittenhouse members in the past, Lucy offers an alternative solution: have her grandfather compile many decades' worth of information on Rittenhouse activities, which can be used to arrest the Ancient Conspiracy's members in the present.
- Wham Line: Lucy's mom, "Rittenhouse would never allow me to become sick".