In the high-security confines of Blackstone Penitentiary, two unlikely inmates form a powerful and dangerous bond. Jae-min Kang, a recently incarcerated psychopath known for his gruesome murders, arrives at Blackstone with a reputation that belies his innocent appearance. Tae-hyun Park, a commanding mafia boss serving a 25-year sentence, controls the prison's power dynamics with an iron fist.

Initially indifferent to the newcomer, Tae-hyun becomes intrigued by Jae-min's cunning and unorthodox methods. He offers Jae-min protection in exchange for his unique skills, leading to a complex relationship that evolves from mutual interest to deep connection. As they navigate the constant threats from rival gangs, corrupt guards, and internal power struggles, their bond strengthens, blending manipulation, loyalty, and rare vulnerability.

During a massive prison riot orchestrated by rivals, Jae-min reveals his true, brutal nature to protect Tae-hyun, solidifying their partnership. Despite increased scrutiny and isolation from the prison administration, they carve out a sanctuary within Blackstone, transforming it into their personal empire.

"Locked Hearts" is a dark, intense tale of love and power, exploring the depths of human emotion and the complexities of relationships forged in the most unlikely of circumstances. Their story demonstrates that even in the harshest environments, connection and love can thrive, born out of darkness and cemented by loyalty and strength.