Undercover police officer Reiji Kikukawa, who has successfully infiltrated the Sukiya-kai yakuza syndicate and goes by the nickname "the Mole," finds himself facing a new international crisis. Having risen to a trusted position within the clan, Reiji is appointed as the personal bodyguard to Shuho Todoroki, the powerful chairman of the Sukiya-kai. His mission becomes deeply personal when Karen, Todoroki’s daughter, is kidnapped by the Dragon Skulls, a formidable Chinese gang. The Dragon Skulls have forged an alliance with a disgraced former yakuza member seeking revenge against the Sukiya-kai, and they plan to sell Karen at a clandestine beauty auction in Hong Kong.

As Reiji travels to Hong Kong to rescue Karen and thwart the international conspiracy, he must maintain his cover while navigating a web of dangerous alliances and escalating violence. The situation is further complicated by the arrival of elite police officer Shinya Kabuto, a rising star in the force who is on a personal crusade against corruption and has made it his mission to arrest Reiji, unaware of his undercover status. The operation draws Reiji deeper into the criminal underworld, forcing him to confront not only the Dragon Skulls but also the threat of exposure from a fellow officer. The story builds toward a chaotic and violent climax in Hong Kong, where Reiji must use all his unorthodox skills to save Karen and survive the collision of yakuza, triads, and a rogue police officer. The narrative is noted for its extreme comedic tone, cartoonish violence, and the hapless yet determined protagonist's continued struggles with his lecherous impulses and his awkward romance with a traffic warden back home.

It is important to note that based on the provided search results, "Mogura no Uta: Hong Kong Kyōsōkyoku" is a live-action film, not a television series. It is the 2016 sequel to the 2013 film "The Mole Song: Undercover Agent Reiji," both directed by Takashi Miike.