Major spoilers ahead for HBO's The Undoing.
For weeks, fans having been tuning into HBO’s limited series The Undoing to uncover the truth behind a mysterious murder that wreaks havoc an elite Manhattan community. Each episode was packed with twists and turns, leading us to believe that the death at the center of the story was the result of something wildly nefarious. But as it turns out, the show's biggest mystery wasn't much of a plot twist after all.
The Undoing follows a series of unfortunate events that throw Grace Fraser's (Nicole Kidman) happy life wildly off course. Things are great for the Frasers; Grace is a successful psychotherapist in the Manhattan area, living in a fancy apartment with her husband Jonathan (Hugh Grant) and young son Henry (Noah Jupe). Theirs is a charmed life...until a woman named Elena Alves (Matilda De Angelis) enters the picture.
Watching the show, fans had many theories about who may have killed Elena. We suspected her husband Miguel (Edan Alexander), Grace's friend Sylvia Steinetz (Lily Rabe) and even Grace herself. People even suspected that little Henry was the cold-blooded killer (and for good reason, because the kid was truly doing the most). But the highly anticipated finale proved us all wrong, in the worst way.
Initially introduced to the PTA of well-off moms during a particularly uncomfortable planning meeting, Elena doesn't really fit in with the community, but Grace does her best to be cordial with her even as the young woman gives off bad vibes. Like, seriously bad vibes.
Unfortunately, Grace's efforts to make Elena feel welcome are in vain because someone doesn't want her around. The young mother is found dead after a brutal attack, and the police are positive that Jonathan has something to do with it. As the authorities build their case, Grace struggles with the disturbing revelation that Jonathan was romantically involved with Elena — her newborn baby is his child — and that their picture-perfect relationship is a lie.
In a last-ditch attempt to maintain his image as husband and father of the year (despite being a whole cheater), it's revealed that Jonathan really is the one who bludgeoned Elena to death with a hammer. Realizing that her husband was not at all who she believed him to be, Grace had to cut herself off from him emotionally and physically — hence the "undoing." Even after being outed as an emotionless killer in court, he couldn't stand the idea of paying for his crimes in front of a court; he kidnaps his son and considers death by suicide until the authorities and his wife intervene to save the son.
Making Jonathan the murderer was definitely something that no one saw coming, mostly because it was simply too easy. He had the most obvious motive to kill Elena, so fans just automatically and incorrectly assumed that HBO would take a different route. But the ending is true to canon, following the storyline of You Should Have Known, the Jean Hanff Korelitz book that the limited series is based on; Korelitz's work is a cautionary tale about what happens when you don't know the person you sleep next to each night.