The story begins with a grotesque murder in Tokyo's Shinjuku district: a victim's body is found twisted, mutilated, and posed in a bizarre manner, with a dying message written in blood using Korean Hangul. Shortly after, a similar crime surfaces in Seoul, Korea, but with the message in Japanese. These alternating murders (victims tortured via archaic methods evoking historical executions) suggest a perpetrator fluent in both languages and cultures, possibly a Zainichi Korean driven by historical grudges from Japan's colonial era and the Imjin War.
Enter Kang Cheong-do, a stoic, elite Korean detective renowned for purging organized crime from Seoul's streets, earning the nickname "Blue Road" for the cleanliness left in his wake. A master of taekkyeon, a traditional Korean martial art, Kang is driven by an unyielding sense of justice. Upon learning of the Japanese case, he travels unofficially to Tokyo, suspecting a cross-border connection. There, he encounters Inose, a pragmatic Japanese detective from Nishigata Station investigating the Shinjuku murder. Inose has already linked it to a prior Yokohama killing and suspects Zainichi Koreans due to the Hangul clues. Initially, the two clash over jurisdiction and cultural misunderstandings: Inose's ignorance of Korean history irritates Kang, but they form an uneasy alliance for a joint investigation, shuttling between Seoul and Tokyo.
As more murders escalate, alternating between the two countries, the victims appear random at first but are later revealed to be connected to hidden historical secrets, such as politicians or descendants with knowledge of past events. Delving into Zainichi communities, Kang and Inose confront themes of discrimination and diaspora. Inose visits Korea and experiences anti-Japanese sentiment firsthand, prompting him to grapple with Japan's colonial past, including forced labor and atrocities. Kang explains "han" not as endless hatred but as accepting and transcending despair, fostering mutual respect.
Interwoven flashbacks transport to the Imjin War, focusing on a fictional Japanese samurai named Emiri, inspired by historical figure Kim Chung-seon (aka Sayaka), a defector who fought for Joseon. Under Toyotomi Hideyoshi, Japan, fresh from unification after civil wars, invades Joseon, but the campaign drains resources, with lords coerced into participation and families held as hostages. Emiri, initially a loyal warrior seeking honorable combat, is appalled by orders to massacre civilians, including women and children. He betrays his comrades, beheading fellow soldiers and declaring, "I stand with you," before defecting to Joseon. Naturalizing as a "Hangwa" (anti-Japanese Japanese fighter), he adopts the surname Hong and passes down a secret martial arts and strategy book to his descendants. This tome emphasizes the "ultimate technique": instilling greater fear in enemies than they instill in you, drawing from Joseon's defeat due to terror from Japanese cruelty. A folk tale illustrates this, where a tiger fears dried persimmons after misinterpreting a child's reaction, highlighting psychological warfare.
Back in the present, clues reveal the killer employs these ancient torture methods from Emiri's book, pointing to a Hong descendant as the culprit. Tracing the lineage, Kang and Inose uncover the Hong family's tumultuous history: a descendant, Hong Kyung-rae, led a 19th-century rebellion in Joseon, after which the family fled to Russia as ethnic Korean-Japanese hybrids. Under Stalin, they were relocated to Uzbekistan, enduring identity confusion: treated as "Japanese" in Korea, "Korean" in Japan, and "foreign" in Russia. Some descendants abandoned Korean nationality for Japanese, mirroring real Zainichi struggles of belonging nowhere.
The investigation encounters obstacles from a large unknown organization, implied to be high-level figures in Korean and Japanese governments or intelligence agencies, suggesting the murders are a cover-up for national secrets. Kang recognizes the killer's techniques as variants of taekkyeon mixed with Emiri's fear-based methods, realizing the perpetrator is his own childhood friend and former martial arts training partner from a Seoul shantytown, their shared "blue road" of innocent memories.
The conspiracy unravels: the killer, a Hong descendant, was brainwashed and used as a "secret weapon" in a botched plot during Park Chung-hee's regime (1960s-1970s). Desperate to return from Uzbekistan exile, the Hong family was coerced into assassinating President Park, possibly with North Korean or communist ties, as they were labeled "communists" in exile. The attempt failed, but to silence potential leaks, the family, via the killer, targets whistleblowers using Emiri's methods. The victims are officials from both nations privy to the secret, with murders alternating countries to sow confusion and blame Zainichi communities. Twisted by exile and manipulation, the killer views the acts as "justice" or revenge against abandoning nations.
Kang journeys to Uzbekistan to piece together the lineage, leading to an intense action climax with fights pitting taekkyeon against Emiri's techniques. Inose provides remote aid, deepening his empathy for Korean history through his partnership with Kang. Kang confronts and arrests the killer amid Uzbekistan's remnants of the family's suffering, but is briefly detained himself due to the conspiracy's influence. Inose's efforts secure his release, symbolizing emerging cross-border trust.
In the final confrontation, Kang and his old friend reminisce about their childhood "blue road" alley, underscoring lost innocence. The killer is captured or dies, though the overarching "large organization" isn't fully exposed, possibly due to real-world sensitivities around 2002-2003 political tensions, like North Korea issues.
In the epilogue, Kang and Inose reunite, with Kang urging Inose to become a "cool detective" who stands as an equal. Inose gains profound understanding of Korea's enduring pain, shedding his initial ignorance. The story concludes on an optimistic note: the "blue road" evolves from personal nostalgia and Kang's name (Cheong-do meaning "blue path") into a metaphor for a brighter future in Korea-Japan relations, where prejudices dissolve through open dialogue and shared humanity.
