Not four months after the D.C. area spawned a new children's play about the Arthurian legend -- "Merlin and the Cave of Dreams," by Charles Way -- the Kennedy Center has launched another, "The Light of Excalibur," by the Helen Hayes Award-winning playwright Norman Allen. Once again we have the leadership lessons, the wizardly wisdom, the blade that's stubbornly stuck in that rock. But this time there's a twist:

Allen's script centers not on the once and future king but on Agnes, a cranky modern teenager who's transported back in time by Merlin so that her Information Age insight, honed on computer games, can help resolve the snarled geopolitics of medieval Britain. Got a bunch of warring Celts and Saxons on your hands? Try applying some lessons gleaned from 21st-century software. It may help.

If this sounds ridiculous in theory, it remains so in execution, despite the best efforts of director Gregg Henry and his team of excellent actors. Rana Kay hits the right tone and expressions -- sometimes sulky, sometimes smart-alecky, sometimes wistful -- in her portrait of Agnes, whose air of adolescent resentment masks acute sorrow about a mother ill with cancer. There's a definite chemistry between Kay's youthful heroine and the princely Arthur, played by Scott Kerns with quiet charisma that always suggests still waters running deep.

When the two characters sit side by side, commiserating about their parent problems, one really wants this time-travel friendship to succeed. More remarkably, they even pull off a spooky scene in which Arthur teaches Agnes the value of empathy, using the sort of mirror-movement exercise that's taught in basic acting classes.

Similarly one can find no fault with Michael Kramer's enigmatic but trustworthy Merlin, with the sad, regal Uther conjured up by Ralph Cosham, or with the scheming villainess Morgause, played by Tuyet Thi Pham in a melodramatically swirling cape.

Unfortunately, the deftness of these performers can't mask the awkwardness of Allen's story line, which inevitably leads to many self-conscious exchanges about 21st-century slang and mores (in one sequence, Agnes explains to Arthur that the phrase "It's okay" can mean "I feel terrible so let's not talk about it"). Perhaps it is possible to write a take on Arthurian legend that gracefully incorporates expressions like "freakazoid" and "pain in the butt" (the latter applying to Merlin), but the feat has not been accomplished here.

To make matters worse, when the script is not dwelling on clash-of-the-centuries absurdities, it tends to bog down in exposition or sentiment.

For example, the first scene's clunky explanation of Agnes' bitter feelings toward her father, who can't bear to frequent the mother's bedside, translates into an all-too-predictable payoff toward the end, when the girl realizes that her sojourn in the Middle Ages has taught her compassion -- even for Dad.

The work of "The Light of Excalibur's" designers is on a par with the acting. Cynthia Abel's period costumes -- tunics, leggings, etc. -- complement Tony Cisek's more minimalist set, dominated at one point by a fortress and, at another, by a simple projection of a cloudy sky. And Kevin Hill's richly textured sound design, together with Jesse Terrill's Celtic-tinged music, is evocative enough to make one understand, at least for a moment, on a gut level, why our era insists on voyaging repeatedly back to the era of the Round Table.

The Light of Excalibur, by Norman Allen. Directed by Gregg Henry. Lighting, Dan Covey; fight choreography, Brad Waller. At the Kennedy Center's Theater Lab through Nov. 7. Call 202-467-4600.

With Rana Kay, Scott Kerns and Michael Kramer, "The Light of Excalibur" boasts three skillful leads, but they are undone by playwright Norman Allen's time-traveling story.