Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they have started a children's birthday party planning business.   Cakes! Decorations! Party games and favors! The Alden children are having fun helping out the 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?