While the HBO series The Undoing did end on the conclusion that had been practically predicted aeons ago, it was the twist at the closing that had many scratching their heads.
The Undoing would have ended with Jonathan winning, save for Grace’s testimony
Everything goes well in the beginning, Jonathan’s attorney Haley questions Grace and she responds by detailing how “empathetic” and “compassionate” her husband is and how “the idea that he would take [a life] is untenable.”
But soon The Undoing takes a sharp turn. It all goes awry as, during the cross-examination, the prosecutor plays the audio of Grace’s 911 call when Jonathan showed up at the beach house, that negates her prior claims that he has never been violent with her. The questioning then surprisingly veers towards Jonathan’s childhood and details that the prosecutor should never have known– how 14-year-old Jonathan felt neither grief nor guilt when his 4-year-old sister, the one he was supposed to be babysitting, died when a car hit her.
She is also aware that his mother told Grace over a call that her son is “not capable of remorse.” While this doesn’t confirm his role in Elena’s murder, it sure does renew everyone’s suspicions about him, robbing him whatever support he garnered by his charismatic performances in the court.
So, what really happened in the finale of The Undoing?
Grace was already shaken by Jonathan having an affair, his countless lies, manipulations and the fact, that he was intimate with her the night he claims he saw Elena’s mangled body. But the last nail in the coffin is her discovering the murder weapon, a heavy mallet, in her son’s guitar case. Henry confesses that he found it in the outside fireplace in their beach house and fearing that it was used by his father to kill Elena, he washed it twice to remove DNA evidence.
The family brings the mallet to Haley’s notice who advices them to not turn it over as not only would it end the case for Jonathan but also result in Henry’s arrest for obstruction of evidence. An obviously guilty Jonathan tries to pin it on Henry, suggesting that he may have come to know about his affair with Elena and killed her in his rage.
This is the turning point in how Grace sees Jonathan. Till that point, she doubted his intentions but in that moment she knows he is a monster that wouldn’t go above incriminating his own son to save himself. In the very next scene, we see her having a long walk with her BFF Sylvia and while the details of their conversation are not divulged, it is obvious that Grace told her to tell the prosecutor about Jonathan’s childhood and how she thinks she has a narcissistic personality that lacks empathy.
Grace knew that these details would weaken his case and ultimately implicate him, without the hammer being turned in and in turn, protect Henry from any repercussions.