Cakes! Decorations! Party games and favors! The Alden children are having fun helping out at neighborhood birthday parties. But then, a house is robbed — its hidden safe emptied — while Jessie, Henry, Benny, and Violet are downstairs entertaining the young guests! When a second house safe is robbed during one of their parties, the Boxcar Children decide there's a mystery to be solved. Will they succeed before another safe is emptied — and another birthday ruined?