The Oscar-winning director of "Up" will explore the mind of an 11-year-old girl in his next feature, slated for a 2015 release.

ANAHEIM -- Pixar Animation Studios' director Pete Docter — who won an Oscar for Up — said his next film, Inside Out, is “one of the most challenging stories I’ve ever had to put together.”

Inside Out, slated for release in 2015, involves a story that takes place inside the mind of an 11-year-old girl, and emotions such as anger, sadness, disgust and joy are the main characters.

Talking with The Hollywood Reporter on Monday at CG convention Siggraph, Docter said the film required creating two simultaneous stories — what is happening to the girl and what is happening inside her mind. “One story is hard enough,” he said. “This is two stories that need to talk to each other.”

Describing the animation, he said, “The characters are created with this energy because we are trying to represent what emotions would look like. They are made up of particles that actually move. Instead of being skin and solid, it is a massive collection of energy."

“It’s a lot of fun and really takes advantage of what animation can do,” he said of the film, planned for a summer 2015 release.

E-mail: Carolyn.Giardina@THR.com

Twitter: @CGinLA