Fred is a sixth-grader reeling from the loss of his beloved dog, Casey. Every day he walks home from school bouncing Casey's old worn-out tennis ball. One day, the ball falls down a sewer grate, and Fred can't bear to leave it down there. He pries open the grate and stumbles down. Through the sewer, Fred enters a parallel universe: Casey is alive, his mom and sister are happier, and there's a version of Fred who's happier too. Spending time with Casey, Fred feels joy for the first time since his dog's death, but he slowly realizes that the loss of Casey is masking an even greater loss: the death of Fred's father. Fred brings his sister, Izzy, to this upside-down world of lost things in the hope of finding their father and bringing him back. Can everything that is lost be found again?