A Sin To Err is the sixth episode of the first season of the television series Agent Carter.
Peggy discovers the shocking truth about Leviathan but doesn't realize that her true enemies are even closer than she imagined. Meanwhile, Sousa is close to confirming the truth about Peggy and may just put her in the crosshairs of the SSR.
Doctor Ivchenko tells Roger Dooley, Peggy Carter, and Jack Thompson that he was captured by Leviathan and the lives of himself and his family were threatened if he did not cooperate. Carter is interested in the Red Room Academy and presses Ivchenko for answers; Dooley tells her to follow her instincts about the possible infiltration of female spies in the United States.
Carter recruits Edwin Jarvis to help to find flings of Howard Stark. She suspects that one of them would have a scar on her wrist from being handcuffed every night; that woman would be a spy who helped steal Stark's inventions. Carter and Jarvis go to the residences of different women but none have scars. The two go to the residence of Ida Emke where Carter finds a bedpost that was scratched up.
Meanwhile, Daniel Sousa goes to the jailhouse that holds Sheldon McFee and McFee positively identifies Carter as the woman who assaulted him and took the truck load of Nitramene.
Dottie Underwood goes to a dentist office across the street from the New York Bell Company. She kills the doctor, then sets up a high powered rifle in the window directly across from Dooley's office. Ivchenko goes to the window. He sees Underwood; she sends beams of light to inform him of her presence. Ivchenko tries to get Dooley to come to the window, but he will not leave his chair. Ivchenko sends a message back to Underwood; he needs more time to complete his mission, but Peggy Carter is the new target.
Ivchenko begins talking to Dooley about the chief's family life and his past in the military during World War II. Dooley is focused on what Ivchenko is saying; Ivchenko talks in a hypnotic tone as he rubs his ring. Dooley's trance is broken when Daniel Sousa tells him that they need to talk immediately.
Carter and Jarvis go to L&L Automat for a meal when Carter notices that the place is being evacuated. She notices Strategic Scientific Reserve agents from Washington, D.C. there, trying to be unnoticeable. Carter and Jarvis fight their way out the restaurant to have Thompson stop them at gunpoint. Carter knocks him out, but is then stopped by Sousa. She apologizes before running off.
Agent Yauch reports to Dooley that the agents failed to capture Carter; Dooley threatens that everyone there will become grade school hall monitors if Carter is not apprehended. Dooley tells Yauch to babysit Ivchenko while he deals with the Carter debacle. Ivchenko speaks to Yauch in a hypnotic tone while rubbing his ring; he tells the Leviathan operative all he knows about the Howard Stark investigation and his inventions and how to escape the building. Ivchenko then instructs Yauch in how he wants him to commit suicide.
Carter goes to the Griffith Hotel to obtain the Blitzkrieg Button from the wall in her apartment. Thompson, Sousa, and a raiding party pursue. Carter hides outside on a window ledge; with help from Angie Martinelli, she evades capture.
As Carter is leaving the building, she runs into Underwood. The spy kisses Carter unexpectedly; Carter is knocked unconscious. As she is falling, Carter sees that Underwood has handcuff scars on her wrists. Underwood draws a knife when Thompson and Sousa approach; she tells them that she found Carter laying on the floor. Carter awakens while being brought to the car of the Strategic Scientific Reserve agents as the residents of the house stare.
At the New York Bell Company Office Carter tells Dooley that she can explain it all.
In the evening, when Fry wants all the residents downstairs Martinelli knocks on Underwood's door to get her. When she enters the room, she notices it empty.
In the interrogation room, where Carter is handcuffed to the table, Dooley tells his men not to go easy on her just because she is a girl.