After her encounter with Quinn (Angus Macfadyen) at the end of the previous week’s episode of "Chuck," Sarah (Yvonne Strahovski) is reset back to her original super-spy state.

Quinn has convinced her that she is a CIA spy, he is her handler, that Chuck (Zachary Levi) has killed her old team, and that her mission is to retrieve the clean version of the Intersect before Chuck steals it. After that, she has to kill Chuck.

Although she is initially suspicious of Quinn, Sarah eventually believes him, and so accepts the mission. She returns to Chuck pretending to have killed Quinn and escaped, though in fact, she is working for Quinn and has an earwig that allows Quinn to give her clues that help her reintegrate into the Bartowski household. Chuck has his suspicions that Sarah is not quite herself and has a late-night conversation with Ellie (Sarah Lancaster) about them, suggesting to Ellie that he needs to destroy the Intersect since it has ruined his life.

Sarah eavesdrops on this conversation and questions Quinn about why Chuck wants to destroy rather than steal the intersect. She also learns about some of the little things that are making Chuck suspicious. Quinn instructs her to proceed with the mission, retrieving the intersect and then killing Chuck.

Next morning, Chuck holds a briefing for Team Bartowski in Castle, and outlines a plan to infiltrate DARPA and destroy the last remaining Intersect. As the team is leaving, Chuck discovers the Intersect glasses, which Sarah has stolen from him, hidden in Sarah’s bag. He leaves them where they are and they proceed with the mission, infiltrating DARPA using knockout gas to put the scientists to sleep.

Sarah singlehandedly takes out the security team that guards the secure area and then goes back to let the team in. They find the Intersect room, which has at its hub, an old Macintosh computer. Sarah uploads the Intersect to the glasses and then takes Morgan (Joshua Gomez) hostage, saying that she knows what the team has done (based on the misinformation from Quinn).

Chuck pleads with her to reconsider and to remember that their relationship is real but Sarah locks them into the Intersect room, places a bomb on the door and walks away. Before detonating the bomb, she questions Quinn about why they don’t just leave Team Bartowski to be arrested by the authorities but Quinn uses an override and detonates the bomb.

Sarah is furious with Quinn for using his override but gives Quinn the Intersect glasses anyway. However, Chuck has switched the glasses for a fake pair that contain no data –- it turns out that his emotional plea also served as a cover for him to switch the glasses since he has realized that Quinn turned Sarah when she was captured.

Sarah vows to fix her mistake and kidnaps Ellie, instructing Chuck to bring her the real glasses if he wants to see Ellie survive the day. Chuck informs Beckman of Sarah’s plight, and then with her assistance, sets up a trap to capture Sarah. However, Ellie takes matters into her own hands and deliberately crashes the car that she is being forced to drive, knocking Sarah unconscious.

Chuck then takes Sarah back to their dream house and tries to remind Sarah that their relationship was real, telling her about the keystone moments. Hoping that he’s managed to get through to Sarah, Chuck then offers her the choice of taking the glasses or staying with him. At this moment, Sarah realizes that Chuck’s feelings for her are real. However, she still believes the story that Quinn has planted, and picks the glasses, saying “Sorry I did my job too well.”

Chuck, realizing that he has not managed to convince Sarah, tries to take the glasses back, and they struggle, though Chuck is unwilling to fight with Sarah. This infuriates her, and she pulls a gun on him. Just as she’s prepared to shoot him, she sees “Sarah + Chuck” inscribed into a door frame and remembers that she wrote it.

As she ponders what this might mean for her mission, Quinn walks in and admits that the story he gave Sarah was a fabrication. He then tries to kill Sarah, but Chuck puts his body between the bullet and Sarah. He tells her that he’ll be okay since he’s wearing a vest and tells her to run, just as Beckman’s team arrives. When he is found, Chuck tells everybody that Sarah has gone.

Casey (Adam Baldwin) goes to find Sarah at her apartment and tells her that even though they both started off as coldblooded agents, over the years, Chuck’s influence has made them both soft, and the he considers her a friend. He then leaves her with a DVD containing the full mission logs, and she watches how the relationship between Chuck and Sarah has unfolded over the years.

Realizing that everything Chuck had told her was true, she goes to find him and tells him that he believes everything but that she doesn’t feel anything for him anymore. She says that she came to say good-bye and that Quinn has taken away her life, and so she needs to go and find him.

WHAT WORKED
The flashback sequences that were used throughout this episode did a really nice job of capturing the show’s history and essence. These sequences, coupled with Sarah’s reversion to her original state, really highlighted how far all of the characters have come over the series.

It was nice to see all the toys that the DARPA geeks are developing –- the miniature robots with lasers were particularly fun, and it was great to see a glimpse of pre-Intersect Morgan as he played with an invisibility cloak, and to see the interplay between Morgan and Casey.

WHAT DIDN'T WORK
Despite the best efforts of the writers, Quinn feels like a massive anticlimax. I was sad not to see anything of Jeff and Lester in this episode.

GIVING CREDIT WHERE CREDIT IS DUE
"Chuck vs. Sarah" was written by Rafe Judkins and Lauren Lefranc, and directed by Jay Chandrasekhar.