Twelve-year-old Etta Johnson has Loud Days and Quiet Days, and she loves them both. Or at least she did. She's not sure how she feels about anything anymore. Her Quiet Days are coming more often, her mom is acting weird, and strange storms fill the streets of Chicago with smoky purple clouds. All Etta wants to do is work on her comic book about Invincible Girl, an unstoppable hero who never worries about anything. Invincible Girl is brave, daring, and bold—everything Etta wishes she could be.

When Etta and her new classmate, Eleazar, discover a mysterious, magical train at the nearby station, Etta doesn't know whether to be amazed or terrified. But then Eleazar's goldendoodle, Louisa May Alcott, runs onto the train and disappears. To rescue her, Etta and Eleazar will have to find their inner superheroes.

Onboard, Etta and Eleazar discover each train car is its own magical world with individual riddles and challenges that must be solved before they can save Louisa May. But 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.