Although he starred in DUMB AND DUMBER, Jeff Daniels is now arguably playing the dumbest character of his career.

He’s Peter – or at least that’s who the pathetic fiftysomething is claiming to be these days. Once upon a time, he was Ray. But after you’ve had sex with a 12-year-old girl and have done nearly four years in prison for it, you ARE inclined to change your name and start again.

Now, 15 years after the fact – or, shall we say, crime — Una, once the object of Ray’s lust, shows up at his workplace. We immediately see that she hasn’t chosen “Let It Go” as her theme song. Any love that goes wrong can be painful, but this?

Peter whisks her into the office’s lunchroom. Scott Pask’s purposely sterile set includes a poster for “Emergency First Aid” which Peter may well need. His having a heart attack isn’t at all unlikely before BLACKBIRD, David Harrower’s harrowing drama, finishes its 85-minute sprint.

New York first saw BLACKBIRD nine years ago at Manhattan Theatre Club with Daniels and Allison Pill. Now no one’s saying that when BLACKBIRD closed in June, 2007 that producers immediately optioned it but needed all nine years to get commercial financing. Still, it is hardly the type of play that makes investors assume they’ll get rich.

Allison Pill is busy these days making movies (HAIL, CAESAR!) and doing TV (THE FAMILY), so Michelle Williams takes her place and knows how to take center stage, too. Una comes in with the confidence of one who has the law and public opinion on her side. Most will agree that a girl of 12 was not responsible for her actions but a man in his forties certainly was.

At first Una doesn’t do all that much talking, but has stands with arms-crossed assurance. She keeps us guessing precisely why she’s there. After about 10 minutes when she snarls in her raspy voice, “I wanted to pull out your eyes, poke them out, stamp one then,” we get more of a hint.

A hint, however, is all it is. Although there is no stronger hate than a love that turns to hate, Williams suggests that Una may still have feelings for the first man she ever loved. At one point she touches her own left cheek for a split- second, almost as if she were checking to see if this mental holocaust has given her a fever – and if it has, will she ever recover?

Una always seems to rally, though. By the time she asks Peter point-blank “How many other 12-year-olds have you had sex with?” we’re not sure if it comes from a need to punish or from sheer jealousy.

For how innocent was Una back then? As Peter points out – admittedly in between a great many unsure “uhs,” pauses, “ums,” stops and starts — some girls love to display their sophistication by being involved with older men. “The last thing you wanted was to be considered a child,” he reminds her. And when she assertively puts her arms akimbo and lashes back, he tells her “You need help.”

Ray-slash-Peter, heal thyself. Daniels shows that this character knows that his argument doesn’t hold water, not a drop. As Williams decides to sit comfortably in a chair, knowing that the onus is on him, Daniels walks as if he’s avoiding a boxer in the ring. The actor clenches his teeth so much that it’s a wonder he has any enamel left on the top of his bottom row. He lets us see that a now-served three-year, seven-month jail sentence doesn’t end the feelings of guilt, shame and remorse. Those provide a lifetime sentence, and Una’s here to assume the role of jailer.

So many actors who sweat on-stage wish that they could control the flow, for surging perspiration implies the nervousness or insecurity an actor cannot afford to have. Perhaps Daniels is a natural perspirer, but we’d all prefer to think that the sweat that covers his face comes from his getting totally into character. He hyperventilates as if he’s just run up all 354 steps to the top of the Statue of Liberty.

But there’s no liberty awaiting him here. Una taunts, mocks and laughs at him. If we adhere to the time-honored belief that “two wrongs don’t make a right,” we can’t much like her, either. The truth is, she doesn’t like herself, either. Soon we see her hunching her shoulders forward in shame and adopting a posture that suggests she’s ready to be shot by a firing squad.

And yet both Peter and Una rally and occasionally use the pronouns that make us shudder: “we” and “us.” An audience would rather not think of them as a unit. Does each of them use the words in hopes of reuniting? Twenty-nine minutes pass before the word “love” is used – but it does arrive.

If Google were a pay site, Harrower would have gone broke while researching, for he’s undoubtedly found every ramification of adult-child sexual relationships. Harrower fills in every possible detail of what went wrong – and what each of them did wrong. That includes how empowered Una felt by bringing Ray sexual and romantic pleasure as well as mention of salacious Polaroids and devised signals to clandestinely meet and minimize their getting caught.

But caught they were, which resulted in Una’s receiving her own type of sentence: seeing a psychiatrist and dealing with parents who never could view their little girl in the same way. “I cried more than I spoke,” she says, choking back tears. Ray doesn’t say he did, but he does mention that fellow prisoners viewed him as the lowest of the low as they do all child molesters.

Lest it all sound so talky, Harrower allows a couple of moments in which words aren’t used at all; they say quite a bit about what these two are feeling before a most unexpected surprise ending.

Many of us remember taking driver’s ed and being shown dolefully cautionary films about teens who speed, get into accidents and either die or become horribly maimed. Perhaps those unfortunate people who have pedophilic tendencies but have thus far refrained from acting on them should see BLACKBIRD. They – and countless children – could profit from seeing what the future would hold for them if they were to make the same mistake that Ray did.

The millions of others who don’t need this advice? Safe to say that they won’t be entertained by BLACKBIRD in the conventional sense of the word “entertained.” But if being mesmerized and holding one’s breath for nearly an hour- and-a-half is something a theatergoer wants to experience, BLACKBIRD is here to provide it.