Plot summary
Grace Fraser is a successful psychologist who lives in Manhattan with her husband Jonathan, an oncologist, and their young son Henry, who attends the elite Reardon School. Grace helps some of the other Reardon parents plan an auction event, where she meets an enigmatic woman named Elena. Grace continues to encounter Elena over the next several days: once in a gym bathroom, where a nude Elena approaches her and remarks on her "kindness," and later at the auction itself, where Grace finds Elena crying in the bathroom and consoles her.
As she leaves the event, Elena kisses Grace on the lips in the elevator. The next day, Elena's body is discovered in her artist's studio by her young son, also a Reardon student. The police rule the case a murder and question Grace and her friends; Elena's husband is noted as a potential suspect. Grace attempts to reach Jonathan, who is supposedly at an oncology conference in Cleveland, but finds that he has left his smartphone in the nightstand by their bed. Unable to trace her husband's whereabouts, Grace becomes paranoid and experiences visions of the scene of Elena's murder.
Credits
Adapted from Wikipedia contributors, The Undoing. Series, exact episode number and English title matched. Paragraphs formatted; references and illustrations omitted.