Book Four: Duet
Childhood friends with opposing personalities, Mingi and Ryan, dream of becoming rock stars but find themselves on the Infinity Train. They explore the locomotive's inner puzzles — and their own relationship — in order to return home, together.
01. The Twin Tapes
Once great friends, Ryan and Min-Gi's relationship slowly dissolves over the course of years of disgruntlement, culminating with their reunion in the restaurant.
02. The Iceberg Car
Waking up on the train together, Ryan and Min-Gi are met by Kez, whose mistake has left the boys in a predicament.
03. The Old West Car
Arriving in a Wild West town, Ryan, Min-Gi, and Kez attempt to escape the locals.
04. The Pig Baby Car
Ryan and Min-Gi are forced to satisfy Pig Baby with the perfect dessert.
05. The Astro Queue Car
With a growing rift forming between them, Ryan and Min-Gi attempt to convince a bouncer to let them enter the rave.
06. The Party Car
With their instruments back, Ryan and Min-Gi have to play their first show since they were kids, but it's no easy emotional feat.
07. The Art Gallery Car
The boys' disintegrating relationship finally comes to a head when they end up in a haunted art museum with seemingly no exit.
08. The Mega Maze Car
As the duo nears Kez's home, they are forced to contend both with all their enemies and their messy relationship.
09. The Castle Car
Having finally arrived at Kez's home, Ryan and Min-Gi learn of her biggest secret and regret.
10. The Train to Nowhere
Ryan and Min-Gi are forced to face the truth and a final challenge if they ever want to go home.