In this final chapter of the trilogy, the four young wellies find themselves taking on the ultimate task of helping River overcome her fear of math. Coming together with the rest of the hallway crew, they devise a plan to help the little girl at the center of their universe see that her dread of numbers is far from the end of the world. ​But when it comes to the crunch, will they flunk their greatest challenge yet? Or will they figure it all out in the end...