When Buster finds an ancient arrowhead in his mother's garden, Arthur digs up his backyard to find treasure, but his search quickly spreads to the front yard, where his mother told him not to dig. When she arrives home, she is disappointed and grounds Arthur for two weeks. Mr. Ratburn's class is at a medieval fair, competing for a trophy against a very intelligent class (who bear striking resemblances to Arthur and his classmates) led by Mr. Ratburn's former teacher, Mr. Pryce-Jones. Mr. Ratburn's class loses constantly until Arthur beats the other class in a "sword in the stone" challenge by figuring out the riddle the challenge's caller recites. Arthur is crowned king of the medieval fair, cheered on by both classes and congratulated by Mr. Pryce-Jones and Mr. Ratburn. Mr. Pryce-Jones realizes that Mr. Ratburn taught his class something he himself never did: to think for themselves.