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The mermaid girl, Bao Rourou, embarks on a journey to find her father, Hai Ren, but becomes entangled in a murder mystery. Through a twist of fate, she crosses paths with the gifted investigator Gu Xinghe. Forced to become Gu Xinghe's assistant in order to uncover her father's whereabouts, Bao Rourou finds herself caught in a web of intrigue. As Gu Xinghe probes her mermaid identity to find a cure for his illness, their relationship becomes a blend of tension and camaraderie. In a gripping struggle, the duo teeters on the edge of victory and defeat. The naive girl unwittingly becomes an apprentice, while the cunning detective unveils a plethora of clever tricks. Amidst this intricate dance, the seeds of love quietly sprout. A case-obsessed man encounters a mermaid girl, and this peculiar pair joins forces to solve a series of perplexing mysteries. Along the way, they gradually set aside their prejudices, offering mutual support and healing to each other.

With each unveiled truth in the cases they investigate, they discover a profound conspiracy linked to Hai Ren's disappearance. With the help of friends Yi Qifeng and Lu Qing'er, they delve into danger, thwart the conspiracy of the Daye Dynasty's advisor, Feng Xuanzi, and the Crown Prince, and rescue the long-tormented Hai Ren.

As they promise a lifetime together, an impending crisis looms, challenging their love once more. How will this devoted couple face the approaching danger?