Back from work after her whirlwind night of travel, Cassie notices an old postcard on the fridge from Izzy’s parents showing a doorway in Egypt. She realizes she may have wrongly assumed the Book of Doors only works with places she has personally visited. Repeating the book’s idea that “any door is every door,” Cassie takes the postcard to her bedroom and tries to picture the doorway until she succeeds.

Cassie opens a door from her bedroom into a warm, dark courtyard in Cairo and sees the Hanging Church nearby. Looking back through the doorway, she can still see her own lit bedroom, which confirms she has truly bridged the distance. The success overwhelms Cassie: the book can take her to doorways she has only seen in images, meaning the entire world is available to her.

That night, Cassie searches for photos of doorways in places she has never been and uses the Book of Doors to visit them, treating it like an experiment. She jumps to new cities across the US and to locations such as an observation deck above Tokyo, a library in Beijing, and a hotel in Rio, returning home through other doors each time. Cassie finds no obvious limits to what the book can do.

The next evening, Izzy is waiting at Cassie’s apartment, and they speak for the first time since the bookstore. Izzy confronts Cassie for continuing to use the book, insisting it is not safe and warning that there is always a “cost,” while Cassie argues that Izzy cannot know that and resents the fear. Wanting Izzy to understand, Cassie persuades her to come through a door once, despite Izzy’s reluctance.

Cassie takes Izzy to the Tokyo Metropolitan Government Building’s observation deck, where they see the city and Mount Fuji. Izzy is impressed but argues Tokyo exists without magic, and Cassie admits the book is about something deeper: escaping the cruelty of life and claiming a life her grandfather never got. Cassie tells Izzy about being raised by her grandfather after her addicted mother left and later died, how he suffered a long, painful death from cancer without good insurance, and how books were Cassie’s refuge through grief. Cassie insists Mr. Webber was kind and she needs to believe the gift is not evil because it lets her live for her grandfather.

Izzy asks to go home, and Cassie brings them back to New York through a door, then suggests Ben’s Deli. At the mostly empty deli after midnight, Cassie recognizes the same man she has seen before, and he recognizes her too. He approaches and introduces himself as Drummond Fox, insisting he has not been following her but warning Cassie that she is in incredible danger.