In the midst of Cardia Beckford's foggy recollections, she can vaguely remember her father Isaac and the solitary residence where she resides, shunned as a monstrous being by the locals due to a lethal substance within her. Planted in her by her father, her heart—the enigmatic Horologium—possesses the ability to generate boundless power while causing destruction upon contact with her skin. In the heart of London, various factions, including the notorious Twilight terrorist group linked to Isaac, covet the Horologium. Captured by the British military to harness its power, Cardia is rescued on her journey by the debonair thief Arsène Lupin, who jests about stealing her heart. Alongside Lupin and his comrades, Cardia embarks on a quest to unravel the mysteries surrounding Isaac's ties to Twilight, her fragmented memories, and the enigmatic Horologium residing within her.