NEW YORK — Beware of any mentions of reasons to be happy in Neil LaBute’s dark new play, “All The Ways To Say I Love You.” Most of them will be quickly snuffed out by subsequent revelations.

The wrenching solo monologue, presented by MCC Theater and starring Judith Light in a masterful performance, opened Wednesday night off-Broadway at the Lucille Lortel Theatre.

LaBute’s many, provocative works for stage and screen include “In The Company of Men” as well as “Fat Pig,” ”Reasons to Be Happy” and “reasons to be pretty,” which transferred to Broadway. In his new play, LaBute cynically shows no mercy to his schoolteacher narrator.

Leigh Silverman directs Light with careful attention to each trembling crack in her character’s initially confident facade. With disarming directness and simplicity, Light, who has won both Tony and Emmy awards, makes her anti-heroine Faye Johnson seem normal and likable — up to a point.

A high-school English and drama teacher and guidance counselor, Faye begins by describing her rewarding work with teenagers and her resignation to a longtime, unexciting and childless marriage with a husband she loves.

Her anxiety rises as she expands on her past relationship with a particular student whom she claims to have saved, and the predatory duplicity that has profoundly changed all their lives.

The question that Faye recalls a student once asking her — “What is the weight of a lie?” — hangs over the entire monologue. As darker layers begin to emerge, Faye’s tension rises, and Light occasionally exults in near-hysteria, tenses in anguish, or wilts into reflective despair.

LaBute piles on the retributions for Faye’s moral transgressions and the deceit that will burden the rest of her life. Yet despite Faye’s unsettling revelations, Light skillfully keeps our sympathy to the haunting, heartrending end.