Ancient Korea, 1506. The tyrannical King Yeonsan-gun of Joseon is overthrown by his half-brother Jung-jong, whose reign begins with a blood bath. Over the years, traitors plot against him, sinking the kingdom into chaos. In 1528, frightened rumors come to royal palace, regarding a mysterious creature, known as Monstrum by the peasants.

Dae-soo and Mi-ra have Ah-reum when they are 17. Ah-reum is diagnosed with progeria, and as his body rapidly ages, his finds comfort in writing a story about how his young parents fell in love.