In Mozart's "Cosi Fan Tutte," two men test their girlfriends' fidelity, only to find out more than they wanted to know.

In Mark Joffe's Cosi, six psychotics and a would-be theater director try one another's patience, with somewhat more satisfactory results.

Joffe's pretext is slender, though fraught with potential.

Lewis Reilly (Ben Mendelsohn) is hired by a Sydney, Australia, mental hospital to direct a talent show in hopes that performing will help build the patients' self esteem.

Along the way Reilly faces a number of challenges, not the least of which is his assistant director (Barry Otto), a perpetual motion machine who talks Reilly into jettisoning the variety-hour format in favor of a full-blown production of Mozart's opera. The task is a Herculean one at best, given that Reilly is working with a half-dozen severely cracked eggs: an ex-heroin addict, a former lawyer, a practicing pyromaniac, a repressed woman who gives advice on wrist-slitting, a woman who imagines Lewis to be attracted to her and the aforementioned assistant, who suffers from more delusions than most people have pores.

This sounds like it could be lots of fun, and it is, at least until problems arise, first at the institution, which takes a dim view of patients attempting anything substantive, and then at home, where Lewis' relationships with his significant live-in other Lucy (Rachel Griffiths) and best friend Nick (Aden Young) have to endure a few fidelity tests of their own.

It's here that an otherwise entertaining film with a whimsical plot and some offbeat performances becomes so intricate that it's in danger of dying of complications.

What's worse, they're fairly standard complications: Reilly can't commit; Lucy can't support; Nick can't act or direct.

Still, the show must go on, and it does, for both Joffe and Reilly, though events become more predictable and Reilly's irregulars start falling into type: the repressed woman learns to express herself, the voiceless lawyer speaks out, the agitated assistant comes to grips -- momentarily at least -- with the fact that he's gripless.

Ever since Olivia DeHaviland crawled out of The Snake Pit, filmmakers have attempted to mine the rich vein of material mental hospitals have to offer. Some, like One Flew Over the Cuckoo's Nest, are brilliant failures; others, like Awakenings, have offered us some real insight into the human condition.

Cosi, like most, is a mixed bag. Its intentions are good, its performances are solid -- particularly Paul Chubb as the vocally impaired lawyer -- the cinematography is attractive, the pacing is swift and in the end nobody's cured, but all's well.

With a little more depth and a little less breadth, Cosi might have become a minor classic. As it is, there's no shame in digging for gold and striking silver. A little disappointment, maybe, but nothing to not write home about.