Maybe you could quibble with the title of Mike Birbiglia’s latest one-man show. The New One, opening tonight on Broadway at the Cort Theatre, is the same one he debuted last summer Off Broadway (now that would be a silly quibble), and all the bassinets in the world couldn’t contain the TV and movie scripts that got to the subject of panicked, bumbling fatherhood long before Birbiglia.

But it would take someone as dim as one of those sleep-enhancing nursery room lights not to figure out soon enough that The New One, really, is the baby, stupid, not the play. And besides, Birbiglia could name his daughter Little Ricky and still seem fresh. He’s that good.

Normcore to the bone in frumpy khakis, untucked button-down over a t-shirt, white sneakers you might see lined up in a one-style-only discount store, a haircut he might have done to himself – Birbiglia arrives on a near-empty stage. (There’s a standard-issue stool, but the remarkable before & after moment that happens during the play, which I won’t spoil, pretty much vanquishes recall of anything pre-after).

The monologist starts with some terrific observational stuff that lays the groundwork for the main event, and won’t sound a bit funny on this page (the difference between a couch and a bed is that a couch hugs while a bed demands a room named after itself. See?) But it is funny, and all the proof necessary that Birbiglia could have set his loamy, conspiratorial This American Life voice to a traditional stand-up career and been just fine, thank you.

“I’ve lost a lot of great friends to kids,” he says, and if the anecdotes of once-cool adults reduced to blithering bores with the arrival of children aren’t exactly novel, Birbiglia, under the direction of frequent collaborator Seth Barrish, breathes them into life.

By the time of his breakthrough Sleepwalk With Me, in its 2008 Off Broadway iteration as well as the book and narrative feature film that followed, Birbiglia had, to put it mildly, narrow-focused his approach. Sleepwalk With Me detailed Birbiglia’s REM Behavior Disorder, a rare condition so severe that extreme measures need to be taken so he doesn’t crash himself through a second-story glass window – again.

Birbiglia briefly recounts that tale in The New One and tosses in a quick account of a fellow sufferer who dreamed he was fighting a wild animal, only to wake up and discover he’d murdered his wife. No joke.

But that’s only one of the reasons Birbiglia needs convincing to take on fatherhood. He likes his marriage and his career just the way they are, and besides his “boys don’t swim.” Pregnancy will be expensive, require surgery (on him) and could risk passing on any of the various medical realities that have left him and his body in the “loser” area (his word) of life’s gene pool. He’s a cancer survivor, has a generally gloomy attitude and that whole sleepwalking thing didn’t just clear up after he made an artistic and commercial success of it.

Once he and his wife – the poet Jennifer Hope Stein, whose poems are read and is credited with additional writing here – do indeed have the New One, the daughter they’ll name Oona – Birbiglia will sleep in a separate room, chain-locked and dressed in a binding, hand-tailored sheet contraption to keep him from fighting wild animals, as it were. Again, no joke.

The emotional heart of The New One, though, beats loudest after the actual new one arrives, and Birbiglia’s physical isolation becomes a metaphor for his emotional distance from mother and daughter. To say he’s not particularly good at sharing or dealing with isolation is an understatement, and leads to one of the play’s two most startling moments (the one that’s not the before & after visual change-up, which, again, I won’t spoil except to applaud set designer Beowulf Boritt).

“I get why dads leave,” Birbiglia confides at one point, and he’s not kidding. The line prompted a much-discussed rejoinder from a New York Times critic: “I get why moms get divorced.” That’s not an unfair rebuke – the darkness beneath Birbiglia’s amiability is his comedic fuel, but probably easier to take from the safe distance of a stage. His envy of the mother-daughter bond feels genuine (he recalls catching his wife sobbing over the fact that Oona will never again be inside of her), but not just a little myopic. Certainly non-biological parents will have some thoughts on the matter. It’s a subject worthy of The Next One.