Lady Seo Hya-na, a noblewoman secretly writing revolutionary poetry, falls in love with Min Yoon-Seok, a scholar-physician aiding the resistance. Betrayed by her fiancé, Hya-na is accused of treason and sentenced to d*th. Yoon-seok, attempting to save her, is fatally wounded. Before dy**g in each other's arms, they vow to find each other in another life.

In present-day Seoul, You are a bestselling poet haunted by vivid dreams of your past life, writes a book titled The Moon’s Scarlet Promise, based on emotions and memories you can’t explain. You're skeptical of love until you meets Min Yoon-Gi, a renowned neurosurgeon who has always been drawn to old poetry—especially ones written by Cheongha, your pen name from the past life.

You being the bestselling poet, are invited to a literary event at a historical palace-turned-museum. There, an exhibition on ancient Joseon-era poetry is unveiled. While browsing, you notice a framed parchment with an old poem signed under the pen name Cheongha. The moment you read it, your vision blurs, and a faint whisper echoes in her mind—your own voice reciting the poem in a language you shouldn’t know.

Suddenly dizzy, she stumbles and is caught by none other than Min Yoongi, a visiting neurosurgeon attending the same event. Your eyes meet, and for a brief second, an unexplainable sense of déjà vu washes over both of you.

Next day you decide to visit an antique store recommended by a friend. As you browse, you come across an old wooden box with an intricately designed hairpin inside. The moment your fingers touch it, a flood of emotions and fragmented images—moonlight, blood-stained hands, a man calling your name—flash before you.

Behind you, a voice asks, “Do we know each other?” as you turn around only to find Min Yoongi, who happened to enter the store at the same time. He’s stares at you with an odd expression— the moment he sees you holding the hairpin, his heart ached inexplicably.