One of the Monty Python team's favored ways to end a sketch was to have Graham Chapman don military garb and declare that matters had become too ''silly.'' Alas, Chapman might have approved of Clay Aiken's stint in Spamalot, the hilarious, Tony-winning musical version of the 1975 movie Monty Python and the Holy Grail.

The American Idol runner-up — appearing until May 4 as the cowardly Sir Robin — is too tentative, withdrawn, and, well, un-silly for a gloriously berserk retelling of Arthurian legend that features vicious hand puppets, absurdly rude Frenchmen, and fourth-wall-demolishing riffs about the Great White Way. To be fair, Aiken handles Robin's big ''You Won't Succeed on Broadway'' number well enough that it doesn't become a self-fulfilling prophecy. He also has a good deal of fun in two smaller comic parts, the idiotic Guard 1 and Brother Maynard, keeper of the ''Holy Hand Grenade.''

But the true Broadway success is Hannah Waddingham, the British actress now playing the Lady of the Lake. The role made a star of originator Sara Ramirez (now a Grey's Anatomy regular); if there's any justice, it will do the same for Waddingham, whose imperious meta-diva effortlessly mocks the baroque vocal gymnastics found on, say, a certain Fox reality show. She's both sublime and very silly indeed. (Tickets: 212-239-6200) B+

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