Mom and Dad are getting ready to go out, leaving Arthur, D.W. and baby Kate with a babysitter. The babysitter is a surprise, says Dad, but to Arthur and D.W., she's more than a surprise-she's a mystery! Their imaginations run wild wondering who will take care of them for the evening-will it be a pirate, a robot, aliens from outer space-or even worse, Mr. Ratburn? When the doorbell finally rings, their panic turns to relief when they find out that their babysitter isn't such a mystery, after all-it's good old Grandma Thora!