"High School Musical" fans who know Vanessa Hudgens best as the smart and talented character Gabriella Montez will barely recognize her in "Gimme Shelter," opening Jan. 24.

In the movie, based on a true story, she plays a pregnant, homeless teenager fleeing an abusive home and finding little help on the mean streets, yet determined to keep her baby.

“I was completely down for a challenge,” Hudgens said during a recent interview at the Four Seasons Atlanta, one of the stops on the movie’s promotional tour.

"Gimme Shelter" was inspired by the work of Kathy DiFiore, the founder of Ramsey, N.J.-based Several Sources Shelters. The network of five shelters serves young mothers and mothers-to-be in crisis. To prepare for the role as Agnes "Apple" Bailey, who ends up at DiFiore's original shelter, Hudgens spent time there.

“I loved building a relationship with her,” Hudgens said. “In the beginning, it was kind of a shock for me. I’ve lived a pretty sheltered life for the most part. Being around young teens, 15, 16, who are pregnant or just had their babies, was a wake-up call. Once I got over the shock, I tried to make myself one of them.”

Ann Dowd plays DiFiore in the movie. Apple's estranged parents, a drug-addled, abusive single mom and a long-gone father who's married and prosperous, are played by Rosario Dawson and Brendan Fraser. James Earl Jones plays the wise and compassionate priest who takes Apple to the shelter.

Hudgens gained weight and chopped off her hair for the role as the wayward Apple.

“I wanted to really change everything about who I was,” she said. “It was a conscious effort to make Apple stand the way she did. I loved the transformation.”

Also known for her work in feature films including “Machete Kills” and “Spring Breakers,” Hudgens will be seen in the future in the campy “Kitchen Sink.”

“Zombies, vampires and humans live together as best they can,” was how she explained the plot.

For now, she’s proud of her work in “Gimme Shelter.”

“It’s been awhile since I filmed it,” she said. “I almost didn’t recognize myself up there. I feel like I was a vessel for the story.”

DiFiore has been sheltering expectant teens since 1981, and the movie based on her experience is meant to raise awareness about the shelter’s mission and the issue of teen pregnancy.

“It’s happening right now, much more than we have our eyes open to,” Hudgens said. “This movie I’m so passionate about. I want it to have a shot. It’s more than just ‘my movie.’ It’s a movie about these girls.”