PARK CITY - Imagine you worked at a Hollywood studio and someone were to pitch you a movie set in the late '70s centered on a clinically diagnosed manic depressive raising his two young daughters all by himself. Your first thought would be to immediately question its commercial viability. Happily, Maya Forbes' directorial debut wasn't dependent on a studio. If it had been, there's no way this wonderfully unexpected tearjerker would have found its way to the big screen.

Inspired by Forbes' own childhood, "Infinitely Polar Bear" immediately introduces us to Cameron (Mark Ruffalo), a husband, a father of two beautiful girls and a man in the middle of a complete nervous breakdown. After spending a few months away for treatment, Cameron tries to reconnect with his sympathetic but realistic wife Maggie (Zoe Saldana) and daughters who both adore him as much as they fear him. With Cameron unable to work, Maggie decides her best option is to get her MBA at Columbia. That means, however, the only person who could take care of her elementary-school-age kids is the same bipolar/manic husband. Initially, this seems like it can only lead to tragedy, but Forbes does an expert job at convincing us of just how much Cameron really loves the girls. Forbes keeps a light hand (similar to Noah Baumbach's "The Squid and the Whale") that still makes it all seem oh so real.  For 18 months, Cameron will watch the girls during the week while Maggie comes back for the weekends. Could this possibly work? You will believe that it does because of Ruffalo, who, frankly, has never been better.

Seemingly in perfect sync with Forbes' direction and screenplay, Ruffalo turns Cameron into a well-rounded, three-dimensional character. This is a performance where even Cameron's anger always seems to hide a smile. This may sound saccharine in the context of this review, but on screen it's anything but. Ruffalo and Forbes also play with Cameron's manic tendencies to show the benefits of such an energetic dad with two adventurous kids. How this is set up through the film's first two acts is the only way the key decision made during the emotional climax will work. And, trust, this scenario could have gone terribly off the rails, but thanks to Ruffalo, Forbes and the best role Saldana's had in years, it doesn't.

Special credit has to go to newcomers Imogene Wolodarsky and Ashley Aufderheide who play Cameron and Maggie's challenging daughters. These girls talk back to their dad, let him know how embarrassed they are of his behavior in public and they push his buttons. If you have any kids over the age of 5, this likely all sounds very familiar. But Forbes does a wonderful job of capturing their true love for him. Whether he spends all night making his first flamingo skirt for one of the girl's costumes or battling with his hoarder-like tendencies, no one will have any doubt of Cameron's true intentions. Even if he may never get his wife's hand in marriage back again.

"Infinitely Polar Bear" should spark strong acquisition interest and should have strong word of mouth on the art house circuit.