GREAT UNSECRET SHOW

Go to the Great and Secret Comedy Show as soon as possible. The Walsh brothers, the amiable hosts of the late-night ImprovBoston show, may soon leave for bigger things in Hollywood or New York. No more free shows. No more free beer. Worst of all, no more Walsh brothers — no more of the amazing alchemy that makes a couple of Charlestown underachievers standing around on stage so astonishingly funny and winning.

The humor is more or less irreproducible. That is, the brothers can reproduce it, but it’s pointless to try to share a Walsh brothers joke on paper: One-liners and jokes are the least part of their act, and their personality is the rest. No theater company will be casting Great and Secret comedy franchises like they do Blue Man Group.

David shuffles around the stage, chews on his thumb, looks thoughtful, smiles wryly. Chris struts more, yells, leaps, being more prone to physical comedy, and his smile is wider, more boyishly enthusiastic. They tell stories about growing up, their life now, what they’ve been doing.

And, barring a bit of standup and sketches from guest comics, and even more rarely, very funny sketches from the brothers, that’s pretty much the show.

It’s great that comedians come to The Great and Secret Comedy Show to try out material, but their desperation when a joke or series of jokes go poorly only makes what the Walsh brothers do all the more amazing. While more traditional comics hone material from performance to performance and eventually arrive at perhaps 15 minutes of killer material, the Walshes have more than an hour’s worth of new stuff each week, and it takes several shows for bits to reappear.

The brothers host some very funny comics in the more traditional observational vein, proving all the more that not everyone can or should do what the brothers do. It is still striking to see comics get up to do a few minutes of material, some checking their watch or their cheat sheet of topics with sweaty desperation, and compare it with the brothers’ winning shiftlessness. The comics can cover 20 topics in five minutes in a kind of swirling panic, while the brothers come on and riff for an hour with cool ease, keeping an audience not just chuckling but regularly laughing aloud. The comics strain for their material to be funny. The Walshes simply are funny, and they get material out of it. But they’re still funny, it can seem, without material.

The trick seems to be that deceptive ease: The Walshes go low-key and take the pressure off.

What’s deceptive about it is that the Walshes are under tremendous pressure. They’ve commandeered the latter half of their show to recount a meandering story about sneaking over the Canadian border to a comedy convention, having already used more than a half-hour of stage time that night. They rely on fleshing out details of the story, and on each other’s amplifications, to fill an hour — and come out the other side without finishing the story. The audience is so amused, charmed and intrigued by the provenance of what it’s hearing that it leaves pleased, not at all put off by the lack of resolution. If energy lags during a show, the brothers simply talk until they find their rhythm again.

It’s hanging out with and hearing the stories of two enormously likable guys, not a series of hit and miss jokes. And it’s great.

But it’s probably not secret enough. Not if Cambridge is to keep the brothers to itself.

The Great and Secret Comedy show runs 10 p.m. to midnight on Thursdays at ImprovBoston, 1253 Cambridge St., Inman Square. The closest T stop is in Central Square, and parking is a bit hellish. Call (617) 576-1253 for more information, or go to improvboston.com. Suggested donation: $5.