It's Christmas Eve in Ravensburg, and the town is bursting with anticipation for its oldest Christmas tradition. The annual opening of Douglas and Son's Toy Store, home of the greatest toys in the world, is finally here. But this is no ordinary Christmas Eve, and the surprise that awaits them is beyond any of their wildest imaginations: a surprise that threatens to ruin Christmas! Stanley, Charlotte, Gertie and Felix call in a little backup, but will...

When the Claymore Diamond is stolen from Ravensburg's finest jewelry store, Stanley Carusoe gets the bright idea that he and his friends should start a detective agency. Armed with curiosity and their love for math, Stanley, Charlotte, Gertie and Felix race around town in an attempt to solve the mystery. Along the way, they butt heads with an ambitious police chief, uncover dark secrets, and drink lots of milkshakes at Mabel's Diner. But when their...

April and Toad were hoping to go home to New York with their father. But History had other ideas. This time, the magical book sends April, Henry, and Toad to Colorado where they'll face their most dangerous threats yet: grizzly bears, lightning fast trains, and snow. Lots and lots of snow! Can the three friends find the missing pieces to the puzzle... or will they be stuck in history forever?

A kid's detective agency uses mathematics to solve problems.