Sometimes this works very well for Mum, like in the Punch and Judy version of the play-within-a-play in last season's A Midsummer Night's Dream, and, I'll wager, their upcoming Wizard of Oz, but in the American premiere of Alan Bennett's quirky The Lady in the Van, the idea feels forced.

For over 15 years, beginning in the mid-'60s, Bennett became the unwilling caretaker of a foul-smelling itinerant who "always had a great faith in onions" and parks her ancient van in his garden, never to depart. His essay about Miss Shepherd in the book Writing Home became a hit West End play in 1999, starring Maggie Smith as the pungent Miss Shepherd.

Despite peevish Bennett's efforts, the homeless woman becomes known as "his lady" and a social worker pressures him, as her apparent "carer," to initiate a constructive dialogue about her needs. Miss Shepherd sticks stubbornly to her ways, surviving by selling pencils and pamphlets, excreting into plastic bags, and musing about becoming prime minister and running England from her van. Bennett's disgust gradually grows to feelings of responsibility and affection, despite himself, and he also discovers her sad history.

At Mum (and Act II, where this production ran last year), Miss Shepherd is a puppet, a jerky doll with a pinched face obscured by her billed hat's shadow. Maureen Torsney-Weir reads the curmudgeon passionately, her voice frustratingly muffled by Nick Embree's cozy set representing Bennett's study.

Gregg Almquist plays two Bennetts: the elder, struggling to craft Shepherd's story into a play years later, and the younger, also a puppet, who he occasionally bickers with ("That's not how it happened!" the alter ego protests). Neighbors, social worker, doctor and visitors — puppets all (designed by Martina Plag) — are voiced by Jered McLenigan and Genevieve Perrier.

Director Robert Smythe's translation makes sense, casting Bennett's memories as puppets and allowing some fun with toy-sized vehicles, but it's a difficult conceit to maintain. When Torsney-Weir finally enters, big as life, to critique Bennett's version of their story, it's a welcome shock — and leaves us feeling we've been deprived of her simply to fulfill the theater's name.

THE LADY IN THE VAN Through March 18, Mum Puppettheatre, 115 Arch St., 215-925-7686, www.mumpuppet.org