Marital infidelity undoubtedly began shortly after the concept of monogamy first popped into the human brain, and thousands of years later, here we are, seduced by the sordid details of Tiger Woods', John Edwards' and David Letterman's personal lives.

Cheating on a spouse represents betrayal at its most primal level. Even spying for the enemy doesn't get to the gut with such a forceful kick.

This raises two powerful human impulses, revenge and forgiveness, and in contemporary theater they lead us straight to Ariel Dorfman, the Chilean-American writer whose work often addresses them.

Dorfman fled Chile as a young man to escape the brutal regime of right-wing dictator Augusto Pinochet, and the experiences of the thousands of Chileans who were jailed, tortured and killed by the repressive government profoundly influence his writing. His best known play, "Death and the Maiden," explores vengeance in the aftermath of gross human rights violations.

In his 2005 drama "Purgatorio," Dorfman examines betrayal and revenge within the intimate confines of marriage, with a nod to Greek mythology and Italian literature. Next Act Theatre opened a trenchant production of the single-act drama during the weekend.

The setting is a spare and antiseptic institutional room. The show's color palette is restricted to white, gray and black. The only two characters are identified as A Man and A Woman.

An early assessment of the situation suggests we are voyeurs in a mental hospital or prison. The man wears a white lab coat, and he appears to be in a position of authority as he speaks with the woman in Dorfman's dense dialog.

But then the positions flip, and the woman is in the lab coat, the man is anxious and squirming. We come to realize that Dorfman has borrowed from the Medea-Jason myth. The woman has murdered their two children in retribution for the man discarding her for another female.

The play's title is taken from the second section of Dante's 14th century narrative poem "The Divine Comedy." A Man and A Woman in this 21st century work must forgive each other before they can leave purgatory and move on to presumably paradise. Hence the alternate probing and discussion.

It's a fascinating premise that holds us when Dorfman's periodic drifts into Expressionism challenge the audience's ability to maintain a pointed focus.

This is demanding material for everyone in the theater. Angela Iannone and David Cecsarini, working under Mary MacDonald Kerr's clear and confident direction, animate the text with passion and intelligence.

As veteran theatergoers know, Iannone can summon internal fire with almost alarming authenticity, and that energy serves "Purgatorio" very well. But she also shows a reflective side here, which is necessary for the production to reach the play's depth. Her woman seethes with the pain of her husband's betrayal, and it also suffers the torment of what she did to her children.

Cecsarini wisely does not attempt to match Iannone's heat. He convincingly portrays an angry, self-assured man who loses enough of his machismo to display regret.

"Purgatorio" continues through Feb. 21 in the Third Ward's Off-Broadway Theatre.

A Sister Act in Oconomowoc

The Rhode Sisters are the best cabaret act in Milwaukee. Theatergoers know Molly as a terrific actress whose list of credits includes nearly every major stage company in the state, but she also plays the standing bass. Alissa is usually found at a keyboard, serving as music director for musicals.

Because a third sister performs a thousand miles away at Disney World, singer and guitarist Rhonda Rae Busch is sneaked in the back door to impersonate a Rhode for the cabaret shows. Darn close to being a family member, she sang at Molly's wedding last year.

The Rhodes debut their expanded three-act show at the Oconomowoc Arts Center this weekend. They are promising 20 new songs for the 7 p.m. performances on Friday and Saturday.

Eclectic is the best word to describe the sisters' musical style. Clever is the best word to describe the way they present themselves. Their banter is charming and original.

Tickets are $18 and can be reserved by calling (262) 560-3172. Food and beverages will be available.

Briefly ...

Spring Green's American Players Theatre has made a change in its previously announced 2010 outdoor season. Lillian Hellman's "Another Part of the Forest" will replace the William Inge drama "Picnic," which had to be dropped because of rights problems.

Veteran APT director William Brown, who will stage "Another Part of the Forest," said the play is a prequel to Hellman's better known "Little Foxes." Calling the piece "darkly hilarious," he added, "it's one of the first prequels ever written."

"Another Part of the Forest" will open June 26 ...

"Wicked" set a Broadway musical record in Chicago for its wildly successful run that lasted nearly four years, but that apparently wasn't long enough. It will return to the Windy City Dec. 1 for eight weeks.

Group sales begin this month. Watch broadwayinchicago.com for the date individual tickets go on sale ...

 The Milwaukee Rep is extending the comedy "Almost, Maine" through Feb. 14 in the company's Stiemke Theater.