Spoilers for The Undoing Episode 3. There's a new suspect in Elena's murder on The Undoing. Though all signs initially pointed to Grace's husband Jonathan killing Elena, it's now looking like Grace could have been the one to do the deed — or was at least involved in helping Jonathan.In the Nov. 8 episode, detectives play Grace footage that shows her walking near Elena's home on the day of the murder and around the time that Elena was killed. Grace acts surprised, explaining that she just takes walks sometimes and didn't even know where Elena lived. But the police seem to think that she and Elena had more of a history than Grace is letting on. "For all we know, she's an accomplice," one detective previously told Grace's father. Spoilers for the book ahead.This would mark a big change from the book The Undoing is based on, albeit not a surprising one, as the show has already departed from the plot. In the novel, Grace's husband Jonathan does kill Elena. And like in the HBO series, he flees the day her body is discovered. But in the book he never comes back: he simply writes Grace a letter confessing.Undoing director Susanna Bier has already hinted at further changes, which only makes the prospect of Grace being involved more likely. "The best you can do with a good book, if you want to dramatize it on screen, is do something else with it," Bier told Oprah magazine. "By doing that you maintain the actual qualities of the book. But you create something that is possibly different and something in its own right."Perhaps the series will incriminate Grace alongside Jonathan or instead of him. After all, she's been having visions of Elena's final moments play in her mind. That could just be her imagining what happened — or it could be her remembering what happened because she was there. According to Jonathan, he and Elena argued that fateful night, they had sex, he left, and she was dead upon his return. If Grace, who was in the area, had seen her husband leaving the apartment, she could have figured out that he was having an affair and killed Elena in a fit of rage.That Grace has done such a complete 180 since Episode 2 only furthers suspicion. Initially, she was telling people that she thought her husband was guilty. Now she's leading the charge to try to prove his innocence. Maybe she knows he's innocent because it's actually her who's guilty.