Ondal never knows what era he'll wake up in! Through his adventures in the history of the Later Three Kingdoms, will he finally grow into a true leader among the Taehak students? Join him as he explores the story of Gung-ye, who dreamed of a new world where the common people would no longer suffer, and learn all about the fascinating Later Three Kingdoms period! Since its first publication in 2017, the 'Seol Min-seok's Great Adventure in Korean History' series has become an essential read for children in Korea learning about their history. Now, the 18th installment, 'Ondal's Strange Time Travel,' has arrived. In this volume, Ondal has become the representative of the Taehak students, but the head of the academy wants to test whether he truly has what it takes to lead Goguryeo as a future son-in-law of the royal family. Each time Ondal falls asleep, he wakes up in a completely different era. This time, he finds himself deep in a forest, about to be attacked by a tiger—until someone comes to his rescue. That someone turns out to be none other than Gung-ye, the future founder of Later Goguryeo! Born a prince of Silla but unable to live as royalty, Gung-ye is determined to build a world where ordinary people no longer suffer, and he gradually builds his power toward that goal. Although Ondal became the new representative of the Taehak students thanks to his friends' trust, his overbearing attitude has caused resentment among them. Watching the young Gung-ye's genuine compassion for the people and his remarkable leadership, Ondal learns a great deal. But as time passes, Gung-ye—now king of Later Goguryeo—becomes consumed by suspicion and paranoia, driving his people and officials toward ruin. Witnessing this tragic transformation gives Ondal a chance to reflect on his own behavior as well. The 'Seol Min-seok's Great Adventure in Korean History' series goes beyond simply teaching Korean history—it uses historical figures and events to help children discover important values and virtues in an entertaining way. This 18th volume uses the stories of Ondal and Gung-ye to illustrate the turbulent final days of unified Silla and to teach readers about the historical background of the Later Three Kingdoms period. Through the many facets of Gung-ye—a man born with a tragic fate—young readers are encouraged to think deeply about what true 'leadership' really means. Just as Ondal grows through his encounter with Gung-ye, readers of 'Seol Min-seok's Great Adventure in Korean History 18' can grow in both knowledge and character alongside him.