Depending upon where one is in the three-plus hours of Wallace Shawn's "Grasses of a Thousand Colors," the apocalyptic tragicomedy is brilliantly upsetting and endearing, or on the annoying side of weird.

For those of us who can never get enough of the playwright/actor's peculiar joviality and merciless grasp of the depraved economics of self-satisfaction, even the annoying part -- the hallucinatory middle section -- is riveting. For people with a low tolerance for meandering stories about explicit sex and orgies with shape-shifting animals, well, consider yourself warned.

"Grasses," first produced in London in 2009, is having its American premiere as part of the rewarding but too brief celebration of the 40-year collaboration between Shawn and director Andre Gregory. (It is coproduced by the Theatre for a New Audience and the Public Theater.)

In superficial ways, the play is more a bizarre erotic journey than a political work from one of our time's major playwrights of conscience. But Shawn, with his inimitably charming and unsettling way, is telling a big-picture story -- maybe the biggest-picture nightmare -- about the effects of a scientifically created food, "Grain #1." It solves world hunger and increases sexual experimentation, before driving the Earth's creatures to madness and agonizing death.

We first meet Ben (Shawn) at a rolling lectern, wearing a black dressing gown and velvet slippers, pleasantly explaining to us in the small, comfortable theater that he, like the privileged few, just happens to be "born lucky." Given what he calls these "optimistic genes," he set forth to fix society's ills. He fixed the food supply by feeding animals grains made of their own species -- a bit like one scientific explanation for mad cow disease.

As he narrates and lives the story, we meet his three paramours -- all named for colors or animals. Julie Hagerty plays Cerise, his worried wife who moves to the woods. Jennifer Tilly is Robin, a voluptuous floozy with a husky, snuggly voice. Emily Cass McDonnell, as Rose, has a special way with felines.

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They enter through a magic door in a lilac wall covered in roses and bond on a tall plushy sofa. We sit close enough to grin back at Shawn's grin, which, in his singular way, makes us complicit in the horror while shamelessly sharing the pleasure.

WHAT “Grasses of a Thousand Colors”
WHERE Public Theater, 425 Lafayette St.
INFO $86.50; 212-967-7555, publictheater.org
BOTTOM LINE Amusing and horrifying Shawn, but also bizarre