"Hungry Hearts," starring Adam Driver and Alba Rohrwacher, was one of the biggest surprises for me at the 2015 Tribeca Film Festival. The film turned out to be nothing like what I expected, and I couldn't be happier about it. The way director Saverio Costanzo masterfully transitioned the genre from a sappy romance to a gut-wrenching suspense, backed by a surprisingly powerful performance from Driver, makes "Hungry Hearts" one of the must-sees at Tribeca.

It starts off as another quirky New York City love story. Boy meets girl by chance when they accidently get locked inside a one-stall bathroom together in the basement of a Chinese restaurant. Jude (Driver) is an engineer, and Mina (Rohrwacher) is an Italian national working at the embassies. They fall in love, she becomes pregnant, they marry. During her pregnancy, Mina has recurring dreams about a dead deer. It leads her to a fortune teller, who tells her the child will be an "indigo child" - a child meant for great things. Soon Mina begins looking at alternative medicines and diets. It starts with her insistence on having a water birth and then, once their son is born, becomes an insistence on having the newborn adhere to a special vegetarian-like diet meant to purify the body. Tensions grow between Jude and Mina when she refuses to let their now-malnourished infant go outside, let alone see a doctor. Her devotion to her child's "organic" lifestyle begins to border on neurotic, putting her and Jude at odds over how best to care for him. As their son's health continues to deteriorate, they each must decide how far they will go to do what they believe will save their child's life.

Parts of the narrative may seem comical or even satirical to read, but they come across as anything but when you're watching the film. Costanzo sets you up to expect the conventions of a romance-genre film, then he toys with you, slowly changing the mood and the tone into a psychological thriller straight out of a Roman Polanski or an Alfred Hitchcock film. Indeed the scenes in the apartment, gradually made claustrophobic and unsettling by Costanzo's tight close-ups and distorted wide-angle lenses, seem reminiscent in some ways to Polanski's "Repulsion," with the biggest difference being "Hungry Hearts" is grounded in a very real situation. Few directors - not even David Fincher in "Gone Girl" - can smoothly change a film's genre halfway in, so kudos to Constanzo for doing it successfully.

Driver surprised me by how well he acted in the film. Although his performance started off slow for me, he turned out to be perfect for the role of Jude. His character was still the perpetually exasperated and ready-to-go-off everyman you see on "Girls," but he made it work. Driver turned in a truthful performance that let you really empathize with the character and the dire situation he finds himself in. Jude is visibly torn between the naïve optimism he can make his crumbling marriage work and the startling reality his wife is insane.

The character of Mina didn't have a lot of emotional range. At times she felt like a foil character to put the audience in Jude's corner. She had two modes, annoyingly stubborn - you wanted to shake her sometimes - and creepily vacant. It's not a slight on Rohrwacher, who deserves credit for her physical performance, especially with what she did with her weight. The best thing about Mina, and in turn Rohrwacher's performance, was also the most frustrating thing about the character: You couldn't hate her. As psychotic as she was, she truly believed she was doing the right thing by her child.

I won't spoil the ending, but I will say it worked brilliantly. It was definitely a surprise given the way the film began, but it was nonetheless believable thanks to the way Constanzo steadily torqued the suspense in the final act. You might not guess the ending, but you'll certainly feel something coming - even if you can't quite put a finger on what it will be.

Four stars.

*This article is part of HNGN's ongoing live coverage of the 2015 Tribeca Film Festival.