Alone in Kellner Books one evening, Cassie sits at the counter studying the Book of Doors. She lingers over its doodles of doorways and faces and wonders who owned it before her and what happened to them. Izzy’s earlier worry about the risks of using the book briefly returns, but Cassie remembers John Webber urging her to go out and see the world and takes it as a message meant for her.

As Cassie cleans up before closing, she thinks of her grandfather confessing his dream of travel and how he never got to do it. That memory hardens Cassie’s resolve: she will not stop using the Book of Doors, because she refuses to turn her back on “magic and impossibility.”

After locking the store, Cassie uses the back-room door to travel to Venice in the hours before dawn. She steps onto the cobblestones, touches the ground to reassure herself it is real, and thrills at the impossible sight of Kellner Books visible through the ajar doorway. Cassie closes the door and wanders through quiet passageways and piazzas, the emptiness making familiar alleys feel eerie, until she reaches the Rialto Bridge, then St. Mark’s Square. Overwhelmed, she calls out that she is in Venice, then grows impatient for somewhere new.

Cassie finds a small hotel with its lobby light on, takes out the Book of Doors, and opens the door to a side street in Prague. She crosses Old Town Square, walks through the Old Town to the Charles Bridge, and remembers a past sunrise there with other tourists, an easy freedom she hasn’t felt in years. Telling herself that the Book of Doors makes her free, Cassie climbs toward Prague Castle as the city starts to wake and checks the time difference, realizing she has been walking for over two hours.

Hungry, Cassie repeats the pattern in Paris, stepping out near Gare du Nord in cold drizzle. She sits under an awning at a café, orders coffee and a croissant, and watches everyday lives moving past, realizing she loves witnessing the stories of strangers. Flipping through the Book of Doors again, she notices new sketches and unreadable fragments and wonders if the book changes. After paying, Cassie returns to the hotel door and opens it back to her New York bedroom; a nearby couple in Paris may glimpse the book’s rainbow light, but Cassie closes the door quickly. She falls into bed exhausted and elated, clutching the Book of Doors, and the next afternoon at work Mrs. Kellner comments that Cassie looks half dead; Cassie says she is fine and was simply up late with a book.