You don’t have to know anything about math to appreciate “Proof,” directed by Mark A. Stutz at Pennsylvania Playhouse, Bethlehem.

The stage play revolves around mathematics and brilliant University of Chicago professor Robert (Dale Beltzner), who has descended into mental illness. His daughter Catherine (Gabrielle Hondrop) has had to drop out of school to care for him.

Her sister Claire (Brooke Harrsch) has flown out to visit her. Meanwhile, Catherine becomes involved with recently graduated Hal (Andrew Maldonado), who was a student of her father and has great respect for him.

The scenes move back and forth in time using just one set, the back porch of Robert’s house.

With only four characters, the play is based on character development and interaction, with a bit of humor here and there. The actors superbly capture the many different facets of their roles.

At first, Robert appears reasonable and upbeat. Catherine is listless and withdrawn, although she seems to have a warm relationship with her father. She is admired from afar by self-admitted nerd Hal, who comes to the house to study her father’s notebooks. She comes to life after she responds to his interest.

Claire visits to prepare the way for Catherine to live near her after she sells their father’s house. She nervously describes how perfect life will be in New York City. Catherine resents her dominance, but does not seem to have an alternative.

As the play moves forward, including flashbacks, Robert is shown as increasingly manic and out of touch with reality. He fills many notebooks with illogical ramblings.

Unexpectedly, Hal finds a brilliant, revolutionary mathematical proof in one of the notebooks. However, this causes a rift between him and Catherine.

The plot is simple (with a surprise at the end of one of the acts) but the relationships are complicated. Claire cares about her sister but alienates her by trying to take over her life. Catherine is disappointed at having to leave school and is worried about inheriting her father’s madness. Hal takes a wrong turn with his academic ambition and by not trusting Catherine.

The play, which received the 2001 Pulitzer Prize for Drama and the 2000 Tony Award for Best Play, is long at two-and one-half hours, but its intensity and the expert portrayals will absorb your attention the entire time.

“Proof,” 7:30 p.m. Feb. 9, 10, 3 p.m. Feb.11, Pennsylvania Playhouse, 390 Illicks Mill Road. Bethlehem. Tickets: 610-865-6665, https://www.paplayhouse.org/