By the end of the first act of “The Children’s Hour,” Lillian Hellman’s once-controversial drama now at the Opera House, a mystifying portrait of a vicious boarding school girl named Mary Tilford emerges. Her malicious powers, mostly used to break rules and blackmail classmates, come to full guile when she unleashes a cunning vengeance on the two women who run a New England school for girls. In one fairly swift blow (she accuses them of lesbianism), Mary sets off a rush of paranoia and brings her teachers mercilessly to their knees.

Even though it was banned in some American cities and in Britain, “The Children’s Hour” was an immediate hit in New York. While the critical world focused on the theme of lesbianism, Hellman, who was 27 at the time, argued that it was actually about the evil workings of slander. She had written the play in 1934 after reading about a lawsuit in Scotland. “I never see characters as monstrously as audiences do,” she said when it was clear that popular opinion veered greatly from her own.

Hellman had been a theater reviewer in New York City but “The Children’s Hour,” which was her first hit, would establish her as one of the country’s highest paid, not to mention prickly, writers of her time. “The Little Foxes,” Hellman’s most famous work, followed in 1939.

“The Children’s Hour” is one of those expertly crafted three-act plays with a perfect curve of drama. One of the dangers of producing such a mannered piece, however, is that it is dated. Penobscot Theatre director Mark Torres rightly points out in program notes that stories about powerfully neurotic children and mass hysteria are still very much a part of contemporary life. While the language and prudishness of this piece may seem a bit overly tailored to modern audiences, the themes couldn’t be more cogent.

One of the other dangers of doing Hellman’s play is the looming cloud of melodrama. If the cast isn’t completely compelling, this story can slip dangerously close to caricature and lose its stirring qualities. When the show opened in the Opera House last week, the cast got off to a bumpy start and only intermittently recovered its balance in the three-hour performance.

Kate Kenney’s Mary Tilford is spunky and pert, but she starts big and has no choice but to grow too big by the time her character leaves stage for good.

Jennifer McEwen, as the teacher Karen Wright, has the stillness and composure necessary for the role, and Kristen Williams, as her colleague and alleged partner-in-crime Martha Tobie, is nearly swashbuckling. They both jump onto stage busily, but their momentum was strained opening night and without that, the story can quickly deflate. It doesn’t help that Rich Upgrove, who plays Karen’s constant but curious fiance, seems to have walked cheerily out of a sitcom rather than a stage drama.

While Ann Foskett is perfectly theatrical in the role of Martha’s loopy aunt, her performance was choppy. Alison Cox, as Mary’s high-society grandmother, was serious and grave, and Kathleen Lake, in the minor role of her maid, gave one of the most naturalistic performances of the evening.

Even the best of actors would struggle to bring believability to this script. It’s a hard one, and, if opening night wasn’t simply an off performance, the cast, which includes a bevy of young actresses, isn’t up for the demands.

All this aside, Hellman’s tale turns out to have been both cautionary and visionary. Not long after “The Children’s Hour” was produced, there would be a massive frenzy over Communism in America. Indeed, Hellman, who leaned far to the left politically, wrote a famous letter to the House committee stating: “I cannot and will not cut my conscience to fit this year’s fashions.”

Her work clearly still has a place in American theater — if not in American thought. But finding the right cast to fit the fashion of Hellman’s conscience is a mighty challenge.

Penobscot Theatre will present “The Children’s Hour” 7 p.m. Thursday, 8 p.m. Friday, 5 p.m. Saturday and 2 p.m. Sunday through March 19 at the Opera House, 131 Main St. in Bangor. For information, call 942-3333.