New Hampshire Theatre Project starts off its winter season with an original, and completely refreshing, new musical, “Mitty!,” by inspired writer and Exeter resident Becky Shepard. Yes, that’s the same Walter Mitty created by novelist James Thurber, an incredibly boring, wishy-washy character who lived a full fantasy life in his mind and nowhere else.

Peter Motson, as Mitty, is one of those very rare actors who steps into the role so thoroughly that you believe every blink, every step, every word. He’s so unassuming and gentle that we feel for him through every battle he wimps out of, praying he’ll be stronger for the next one. And as a song and dance man, he charms; all in all, Motson doesn’t just impress, he gets the audience rooting for underdogs everywhere.

Mitty lives with his faithful wife Agnes (Genevieve Aichele, with a golden voice), his overbearing mother (Kathy Somssich) and his two children (Jessica Noone and Jake Pleadwell, who are just as talented as their “mother”). As Mitty fails them one by one, he only has one being he can turn to: Delilah. Shepard took a bit of poetic license and added this character of a spunky and loyal dog who is Mitty’s constant companion. Meghann Beauchamp is thoroughly convincing from bark to bite. Next-door-neighbor Milton Bustard (the chameleon-like Tim Robinson, in one of his several delightful roles) takes advantage of Mitty’s nonconfrontational nature by insisting he cut down a tree that’s dropping apples onto the Bustard lawn, despite the fact that it has great meaning to the whole Mitty family. The entire family captures the “gee whiz” quality of the early 1950s beautifully.

Aichele is sweet yet strong, embodying the perfect wife, and the tension with the domineering Somssich reminds us of every uncomfortable family dinner we’ve ever suffered through. But in the end, the Mitty marriage is so enduring that even Tiffany Baldridge (Teighe Thorsen), gorgeous, seductive and the owner of Sampson, Delilah’s new love, can’t tear them apart. (As a side note, Thorsen, who has a figure that most women would kill for, writes in her bio that she works her size 8 body as a Plus Size Model, an remarkable comment on our culture’s expectations of women.)

Kate Kirkwood’s tone-deaf opera singer was a hilarious bit of comedy, done perfectly. Another delightful point was the chorus, who became members in Mitty’s movie premiere, his boxing match, his grand home-coming as a ballplayer; they also continually urge the man to stand up for himself. They’re with him always, and try to guide him well.

The songs are upbeat, catchy and fun, but on several occasions the accompaniment doesn’t keep up with the vocalists, which is distracting at several points. But it’s quickly rectified by the performers, who go to it with gusto. Though not every actor has a Broadway-quality voice, they all sell their songs so completely in character that it’s beautiful. And, I’m being really picky here, I felt that Walter’s fantasy life was referred to far too often in the text; his distractedness was what made his wife crazy, not his dreams.

All in all, for a world premiere performance, “Mitty!” is a great way to spend a winter evening. Don’t be surprised if you find yourself singing “It’s a Fact, Walter Mitty” on the drive home.