What if we had little people running around in our heads directing the action? Joy, Sadness, Anger, Disgust, and Fear all pushing buttons. Forcing issues. At least you'd then have someone to blame, right?
Inside a little girl named Riley are the personifications of the five base emotions. They've grown up with her, formed her personality, and helped her maintain her composure. First among them is Joy. Joy dominates to a degree far above the others, making the best of every situation to the detriment of Sadness.
Riley's family moves from wintery land (I forget where but there was hockey) to San Francisco. With all the changes, Joy begins to lose control. Forcing the issue, her and Sadness are cast into the mind out of the control room. They must fight their way to get back, finding old imaginary friends and riding trains of thought. The movie's real clever.
The central theme of the movie is embracing sadness. It sounds like a bad theme, but it works. Throughout the film, Joy forces the issue. Hides behind platitudes and tries to protect old happy memories from turning sad. But then we learn without the sadness, how do we know about the joy? We learn depression is not the absense of happiness or overwhelming sadness, but a shutdown of all emotions. That the most dangerous thing the human mind has to fear is itself.
Also, it made me care about a goddamn thing monster called Bing Bong. So it's got that going for it.