Twelve-year-old Etta Johnson has Loud Days where she can hear just fine and Quiet Days where sounds come from far away and she gets to retreat into her thoughts. Etta spends most of her time alone, working on her comic book about Invincible Girl, the superhero who takes down super villain Petra Fide. Invincible Girl is brave, daring, and bold--everything Etta wishes she could be.

But when Louisa May Alcott, a friendly Goldendoodle from across the street, disappears, Etta and the dog's boy, Eleazar, must find their inner heroes to save her. The catch? Louisa May has run onto a magical train that mysteriously arrived at the station near Etta and Eleazar's houses. Onboard, they discover each train car is its own magical world with individual riddles and challenges that must be solved before they can reach the engine room and rescue Louisa May.

Only, the stakes are even higher than they thought. The train's magic is malfunctioning and spreading a purple smoke called The Fear through the streets of Chicago. Etta and Eleazar are the only ones who can save the city, save Louisa May Alcott--and save each other.