Spoiler alert! We're discussing major details about the plot and ending of “Mothers’ Instinct” (in select theaters now; available Aug. 13 as video on demand).

Some people would kill to be parents.

That’s the deliciously bloody premise of “Mothers’ Instinct,” a suspenseful new thriller in which Anne Hathaway goes toe to toe with Jessica Chastain, playing next-door neighbors whose friendship sours after the death of one of their young sons. The twisty period drama has everything we could want in a movie: a pair of Oscar-winning actresses chain-smoking in fabulous wigs, while also digging their teeth into a meaty story about grief and revenge.

The movie is based on the 2012 novel "Behind the Hatred" by Belgian author Barbara Abel, although substantial changes have been made in the film adaptation. Here are some of the biggest differences between the book and movie:

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The movie 'Mothers' Instinct' changes the character names and the setting

For starters, one of the major differences is that the film is set in a 1960s suburban American neighborhood, whereas the book takes place just outside Paris in the recent past, with references to watching “SpongeBob SquarePants” and playing Nintendo games. The characters’ names have mostly been changed for the movie as well: Laetitia is now Alice (Chastain), Tiphaine is now Celine (Hathaway), and so on.

Max's tragic fall happens very differently in the 'Mothers' Instinct' book

Early in the book, Celine’s son, Max (Baylen D. Bielitz), stays home sick from school. Running a high fever and struggling to cool down, he opens his bedroom window and leans out too far, leading to his tragic death. But the circumstances around his fall are vastly different in the film: Max is still home ill, but goes out to the balcony to fiddle with a birdhouse in a nearby tree. Walking on top of the railing, he loses his balance and plunges to the ground.

In both versions, Alice is in her backyard next door just before the fall occurs. Unable to get through the large hedge separating the two houses, she runs to the front door of Celine’s home to grab help but is sadly too late. Many chapters of the book are devoted to whether Alice could’ve saved Max by breaking his fall, and how she might’ve gotten to him faster had they installed a pathway connecting both yards. Fortunately, most of the landscaping disagreements have been pruned for the film.

Jessica Chastain's 'Mothers' Instinct' character has a richer backstory

The movie smartly fleshes out the character of Alice. In the book, she loses both her parents in a car accident before meeting her future husband, Simon (Anders Danielsen Lie), who helps her work through the grief. But the film gives a deeper explanation for Alice’s paranoia around Celine, who takes an uneasy interest in Alice’s son, Theo (Eamon Patrick O’Connell), after Max’s death.

As we learn in the movie, Alice feels immense guilt for being the only survivor of the car crash that killed her parents. She suffers from anxiety, is prescribed medication, and quits her journalism career after multiple stays in mental hospitals. As a result, Simon is skeptical when Alice starts lobbing accusations of murder against Celine, wondering if it's just another psychotic break. The story also becomes more suspenseful for the audience: Is Alice an unreliable narrator, or is she right to fear Celine?

'Mothers' Instinct' ending has major differences from the book

The film’s last 30 minutes make numerous changes from the source material. In the book, Celine is a former pharmacist who now whips up herbal remedies at home – one of which she disguises as Doritos dust, which she uses to try and poison Theo. But in the film, Celine’s husband, Damian (Josh Charles), is the one who works in pharmaceuticals, and Celine instead tries to weaponize Theo’s peanut allergy by feeding him peanut butter cookies.

In the book, Simon also has a long backstory of being an ex-con and recovering addict. His sponsor, Ernest, becomes Theo’s godfather and is Celine’s first victim. But all of that has been cut from the film: Instead, Celine kills Alice’s mother-in-law (Caroline Lagerfelt), to ensure that Theo has no close relatives who could look after him, should his parents mysteriously die.

Anne Hathaway's 'Mothers' Instinct' character is strangely more sympathetic

Celine has a slightly higher body count in the movie. In the novel, she and Damian work together to kill both Simon and Alice: stealthily framing both their murders as suicides, and eventually gaining custody of Theo in the book’s final chapter. But Celine works alone in the film: First, she kills Damian in his sleep, making it seem as if he slit his wrists. Taking pity on her, Alice invites the newly widowed Celine to stay at their house. One night, Celine chloroforms both Alice and Simon and stages a gas leak to make their deaths appear like an accident.

The book ends on a sinister note, with a child therapist telling Celine and Damian that Theo is lucky to have them as parents. But the film has a more sentimental finish, with Celine and Theo walking gloomily together on the beach. Tearfully, she reassures her new son that while they may both feel broken inside, together they can help each other heal. With a faint grin, they begin to run and play on the sand before the end credits roll.

For all its high-camp moments – of which there are many – the movie ultimately works better than the book on an emotional level. Hathaway and Chastain are both acutely attuned to the film’s pitch-black humor and Hitchcockian melodrama, and achingly convey their characters’ desperation and resentment. It turns out, there are no limits to a mother's love.

Now, who’s hungry for some peanut butter cookies?