No reason is suggested for Justine’s (Kirsten Dunst) emotional roller coaster in Lars von Trier’s Melancholia, though apparently instability is nothing new in her, some sort of mental problem from which many suffer. Chemical/hormonal imbalance nowadays, for the Elizabethans melancholy came from an excess of the bodily fluid or humour black bile. The director-writer’s modern pessimism is here as elsewhere different from that of, say, a Polanski, who laces his abnormal psyches with a sense of puckishness.

The Melancholy Dane’s own script is uninterested in whatever the origins of the perverse apathy that at times lapses into comatose lethargy. It simply is, with stress on result, not cause. The collision-course approach of the title’s greenish Earth-mirror planet elicits others’ responses of fear, wonder, anxiety or depression, but the woman remains unchanged from what she has been. Why in the world she consented to marry innocent love-blinded Michael (Alexander Skarsgärd) is never hinted, nor is there explanation for her about-face from giggly tender to cold distant; she turns off the groom as neatly as she turns down a gift promotion from crass boss Jack (Alexander’s father Stellan) and at once quits her job entirely.

The film world-premièred at Cannes, later to be selected for the Toronto and New York festivals, and blonde curvy Dunst earned Best Actress at the former, but brunette angular Charlotte Gainsbourg could as easily have carried away the palm as her sister Claire.

It must be inherited wealth, for no one actually works, but Claire and husband John (Kiefer Sutherland) live in a baronial stone mansion out of Gatsby, filmed in Sweden, with stables, golf course and cart and immaculate gardens running down to the sea. Permanent staff are few or unseen, but still Claire’s cooking up a special meat loaf treat or anything is an incongruous absurdity. She has convinced her husband to give a lavish catered wedding celebration for the newlyweds, who arrive hours late when a limo cannot negotiate country curves. As in Gatsby, too, the obscenely wealthy couple have a single child (Leo, by Cameron Spurr) to give play to parental and auntly concerns but also to show that his parents have been physical despite an emotional aloofness in their present relationship.

Even superrich John brings up the cost of the wedding do but, knowing the bride’s instability -- “Is everybody in your family crazy?” -- like almost everyone, hopes she will be happy. That almost takes into account her bitter, obscene divorced mother Gaby (Charlotte Rampling), who advocates avoiding marriage altogether and whose customary antisocial behavior results in her tatty suitcase being thrown out on the steps. Debonair father of the bride Dexter (John Hurt) enjoys the drinks and the company of the ladies but opts out when his daughter implores his time, counsel and love.

That leaves Claire, more conventional -- if that can be applied to the fabulously wealthy -- and more conventionally stable, to fuss over her adored sister, bathe and cuddle her, and take her out on dawn horseback jaunts where Justine senselessly whips favorite Abraham to cross a small bridge.

The stallion may intuit something in his rider, or animals are attuned to earthquakes, hurricanes and rogue planets. John, who will prove the weakest, forbids his wife to check online, encourages only Leo to inspect looming Melancholia through a telescope, and twists scientists’ calculations of celestial disaster. The odds of annihilation from the heavens, or simply impact, are small but real. There is no reason the movie menace grows smaller before getting bigger in Leo’s homemade wire measurer, or for it to emerge from behind the Sun and bypass Mercury and Venus to draw a bead on our 3rd rock from the Sun.

Then, there is not much reason for anything in Melancholia. When others fall apart and break down, madness is the only preservative.

(Released by Magnolia Pictures and rated "R" for some graphic nudity, sexual content and language.)