"The Runner Stumbles" at the ArcLight Theater Sex and murder don't guarantee a successful play. But they sure don't hurt its chances. That's certainly true in "The Runner Stumbles," a drama that surmounts more than a few problems to become quite an absorbing show. Set in 1911 and based loosely on real events, the story centers on Father Rivard (Christopher Patrick Mullen), a disheartened small-town priest who finds himself drawn to Sister Rita (Casandera M. J. Lollar), a new nun at his ailing parish. Their relationship becomes a source of gossip, and when Sister Rita is later found dead, Father Rivard is the prime suspect. First performed on Broadway in 1976, "The Runner Stumbles" is feverish with sexual tension and suppressed anger. This revival, directed by Peter Zinn at the ArcLight Theater, keeps the stress high, though time hasn't been particularly kind to parts of Milan Stitt's script. Plays like "Doubt" have better explored the gray area between truth and lies, while programs like "Law & Order" have set the pace for speedier storytelling. Still, there is real excitement in the characters" shifting emotions, and to focus only on the courtroom scenes (or the sporadically clunky dialogue) is to miss some terrific moments. Mr. Mullen is a first-rate actor, alternating between gentle and odious. He uses slight gestures - the handling of a book, a glance into the dark - to great effect. Ms. Lollar is also a multifaceted and forceful presence. While several minor characters are underwritten or overacted, these two leads are exceptional. Even when "The Runner Stumbles" shows its age, it remains a shrewd mystery with a surprising amount of humor. The production, at nearly two and a half hours, sometimes threatens to lose its audience. But by exploiting a plot packed with religious doubts, murder accusations and forbidden love, it reels them right back in.