Stop, children . . . What's that sound?

When it comes to conspiracy theories, most people land somewhere between the extremes. Perhaps they believe that Oswald acted alone, but the Apollo space landing was a hoax. Or they implicate the CIA in killing Martin Luther King, but accept Princess Diana’s death as a tragic accident.

But at the very ends of that spectrum, you’ll find the nuts and the hopelessly naïve: Those who believe that nothing happens without a hidden (and, more often than not, malevolent) subtext; and those who insist on believing that things are exactly as they appear, regardless of the contrary evidence.

Yankee Tavern, a new play by Steven Dietz at New Jersey Repertory Company, is all about those extremes, and what happens when the ground beneath closely held positions shifts.

Adam is a serious young grad student, working on a dissertation about urban myths and their role in perpetuating dangerous, elaborate conspiracy theories. He is angling for a job with the CIA when he graduates. Meanwhile, he is stuck tending the bar in his late father’s tavern while he waits for the city to condemn the place. Adam’s fiancée Janet works at an unnamed foundation and is knee deep preparations for their wedding.

The play’s beating heart is Ray, a clever but very crazy homeless man who was once the best friend of Adam’s father. Ray sees conspiracies everywhere he looks. He is chock full of doomsday predictions, unprovable assertions and half-baked speculations about everything from the occult Starbucks logo to the real story behind those notorious hanging chads.

It is the events of September 11, 2001 for which Ray saves his most zealous theorizing. Obsessed by questions about who knew what and when, by which parts of the tragedy constituted coincidence and which were the result of covert schemes, his rantings are eloquent quackery.

But Ray’s crackpot notions suddenly seem less implausible when a mysterious stranger sits down at the end of the bar. In the midst of one of Ray’s rants, the newcomer finishes one of Ray’s sentences. And he proceeds to reveal a surprising intimacy with the details of September 11 and, even more menacingly, with Adam’s research. Suddenly, all of the odd details of the World Trade Center tragedy, which so preoccupy Ray, are seeming much coincidental.

SuzAnne Barabas directs the first act as black comedy. Despite the dark topics, Ray’s rat-a-tat tirades are both silly enough and true enough to be very funny, and even the entrance of a mysterious stranger supplies a kind of Gunfight at the OK Corral campiness.

But that good-natured bemusement is completely absent in the play’s second half. With Adam off to Washington, D.C. for a round of secretive meetings and Ray off in search of a suit to wear to the couple’s wedding, the mysterious stranger returns to find only Janet behind the bar. The simmering menace of the first act erupts into a rolling boil.

Jim Shankman chews the scenery as Ray, delivering twice as much dialog as the other characters in half the time. He manages to make his rants at the same time self-deprecating and egomaniacal, completely conveying the incontrovertible proof of hundreds of conspiracies, everywhere he looks. In the second act when Ray is largely offstage, the play seems to deflate a little.

Despite being deliberately upstaged by Shankman, Pheonix Vaughn and Jason Odell Williams are both very good as the couple whose center cannot hold when their facades begin to disintegrate. Michael Irvin Pollard carries a tortured menace as Palmer, the mystery man who knows too much,

Barabas’ crisp direction steers the play along briskly enough to gloss over any small plot holes. Yankee Tavern is a surprising and effective piece of theater that is both funny and suspenseful.

Yankee Tavern runs through May 23 at New Jersey Repertory in Long Branch, 732-229-3166. www.njrep.org.