Set during the reign of King Sejong the Great in 15th-century Joseon, 'Tree with Deep Roots' chronicles the turbulent events surrounding the secret creation of Hangul, the native Korean phonetic script. When a series of gruesome murders strikes royal scholars at the Hall of Worthies (Jiphyeonjeon), fear grips the royal court. King Sejong, who seeks to empower commoners with literacy, faces covert opposition from the Joseon aristocracy and entrenched Neo-Confucian scholars who view the democratization of the written word as a direct threat to their authority. Entering the heart of the intrigue is Kang Chae-yoon, a low-born royal guard driven by a long-standing personal vendetta against King Sejong, whom he blames for the tragic death of his father years earlier. Assigned to investigate the serial murders alongside So-yi, a mute court maid possessing a photographic memory and deep ties to Chae-yoon's forgotten past, Chae-yoon uncovers the clandestine shadow society known as 'Milbon' (Hidden Root). The secret faction, led by descendants of the dynasty's founding architect Jeong Do-jeon, will stop at nothing to prevent Sejong from subverting their vision of ministerial supremacy. As the intellectual clash turns increasingly lethal, Chae-yoon's quest for revenge shifts into a fight to protect the King's revolutionary enterprise. Balancing intense philosophical debates with gripping martial arts action and detective mystery, the series paints a compelling portrait of King Sejong's visionary leadership, the high price of political reform, and the foundational sacrifice behind the birth of a nation's voice.