The Folger Theater's "Medieval Christmas Pageant" works like a charmed time- machine. You could swear, when it's done, that you've been to another age.

Everything about this show feels authentic, from the actors' first straggling entrance to the snuffing of votive candles. And even when the language is Latin, the doings don't stray from the fold.

The production -- directed by Ross Allen, with sets by Hugh McKay and costumes by Bary Allen Odom -- offers a trinity of medieval Christmas plays (first slapstick, then mystical, then a mix of both) with musical interludes from the Folger Consort, expanded for the pageant to four players. But despite lavish looks -- celestial lighting, imaginative costumes and miraculous special effects -- the show seems disarmingly simple.

The comedy, as in "The Mayor's Christmas Pie," is broad and earthy; the theology, as in "The Shepherds," a 13th-century Nativity play sung in Latin, is no-frills. The mood is that of a block party, as some of our ancestors might have thrown it.

The Folger's resident acting company shows blithe and holy spirit enough for the evening, especially when joined by soprano Ann Monoyios, who plays an angel -- and sings like one -- in "The Shepherds" and "The Second Shepherds' Play."

This last, though somewhat slow-paced in the clutch, also features solid comic performances from John Wodja as a lamb- rustler, Mikel Lambert as his wife and accomplice, and Craig Paul Wroe as the lamb -- in Odom's cleverly stylized getup.

The Folger Consort, medievally garbed in bright red and green, sets the tone with tunes mostly from 14th-century Italy and 14th- and 15th-century England, performed on period fiddles, lute, recorder, psaltery, mandora and kamenga -- and danced to, with energy, by the actors. This early music is spare and sprightly -- often just a melodic line with the barest of harmonies -- and Monoyios' singing is appropriately clear and light. A MEDIEVAL CHRISTMAS PAGEANT -- At the Folger Theater through January 9.