In fifth-century China, fifteen-year-old Wencai aids his childhood friend and secret love interest Yingtai in posing as a boy to attend school together. Amidst the ancient halls of learning, Yingtai falls for Shanbo, their newly sworn blood brother, who reciprocates her affection—albeit for the boy he thinks she is. Jealousies provoke rifts in their “brotherhood,” culminating in Shanbo’s violent death, for which Yingtai blames Wencai. At Shanbo’s funeral, Yingtai leaps into his tomb, triggering a spell that turns the pair into the legendary Butterfly Lovers. Wencai spends the next sixteen centuries tracking the butterflies and battling the Bull King and Witch of Wuxian across China to undo the magic, clear his name, and declare his love for Yingtai.

Meanwhile, in contemporary Shanghai, eighteen-year-old aspiring actor Brandon secures a breakthrough role in a movie adaptation of Butterfly Lovers. During a research trip, he captures a dazzling butterfly, unaware that it will metamorphose into a young lady named Butterfly. Their connection deepens as they find themselves targeted by a shadow organization led by the menacing Scarface, prompting the girl to depart, sacrificing her own safety to protect Brandon.

The vengeful Bull King and Witch of Wuxian capture Yingtai/Butterfly. Wencai and Brandon race to mythical Wuxing Mountain to save their true love but ultimately it falls upon the girl to make a heart-wrenching choice—whom to love and whom to rescue from a fate imperiling them all.