A one-man version of "It's a Wonderful Life" is an unusual, even dubious premise for a show.

But that's not exactly what actor Mark Setlock delivers in "This Wonderful Life." More precisely, the dramatic action of this seasonal attraction at the Public Theater is that of an excited and loving description of a beloved movie, delivered by a new best friend who lapses naturally into impersonation as he goes.

The difference matters. A one-man version of a movie would be a gimmick, remarkable mainly for its technical virtuosity. But a friend telling you about a movie is the most natural thing in the world, something we experience all the time -- except that it's rare to have a friend with Setlock's ease, vivacity, physical discipline and story-telling imagination.

This telling is the show. I hesitate to call it a play; story theater, maybe. The title, "This [sic] Wonderful Life," is probably due to copyright. But I like to think that the slight alteration allows it to embrace also the life of the storyteller, who is having a wonderful time sharing his enthusiasm for (and channeling part of) the Frank Capra classic.

Enthusiasm, sharing, personal engagement and sense of fun -- these are all key to Setlock's performance. They also help shape the script, which feels spontaneous, as it should, since the show was originally conceived by Setlock, who then enlisted the help of playwright Steve Murray.

Setlock begins by artlessly running on stage in casual clothing to say he's going to tell the story of his favorite movie. After a joking two-minute version, partly to put us at ease and partly to stir the movie memories that most of us share, he sets off to do it in detail. The whole thing takes 80 minutes and leaves you wanting more.

The detail is selective, of course. A lot is accomplished not by dialogue but by Setlock's stepping back and describing a scene, often with a wry exasperation at some oddity that makes his affection even more apparent than would outright worship. (If you've seen "Drowsy Chaperone," think of the running commentary of The Man in the Chair.)

He is almost caustic about the movie's ethnic stereotypes (Mr. Martini; Annie, the maid), but he gets no deeper into sociological commentary than his offhand injunction, "Have you noticed how much the story is about money? Discuss."

Aiding him are a few strategic props and furniture pieces, plus a contraption of heavenly lights to visualize the angels, a bridge that rises up from the floor and a Christmas tree that arrives on cue. Some of the characters appear as recorded voices (by Setlock, of course), but he also skillfully carries on conversations between several people at once.

He never changes costume -- instant changes of character suffice. His Potter is great; his Clarence, Uncle Billy and George are good; the rest are OK. Some, like parts of his commentary, tend toward affectionate caricature. I admire the little details he captures, things you might have forgotten until he honors them.

Director Martha Banta and the Public's designers and technicians support Setlock with effects that enhance the story without becoming the story themselves.

Setlock is the real story. We like him and he becomes the attraction as much as the story he tells.