真正的背叛者是誰？以一封密告文為開端的絕密內部搜查與未解决案件的真相相連。跟踪、埋伏、詐騙。充滿驚險和神秘感的冷酷懸疑劇。

　追查超過４萬名警視廳職員不正當行為的警視廳人事一課（人一）監察系所屬警官佐良正輝。人一是為了保護警察內部的紀律而懷疑、監視同一警察官，有時會告發的“警察中的警察”。是一個孤獨而秘密主義的專業集團。

　佐良在某個事件的搜查中殉職失去了學弟刑警，從搜查一課調到了人一。被同事視為叛徒、被敵視的同時也孤獨地工作著，但以一封密告文為開端，被命令監視以前的同事皆口菜子。在推進每個人的跟蹤(行為確認)的過程中，也終於到達了某個未解决事件的真相和警察內部潜藏的深深的黑暗……。

警视厅搜查一课的刑警·佐良正辉，在某个杀人事件的搜查中，因殉职而失去后辈刑警·斋藤康太，被调到了取缔警察职员不正当行为的“人事一课监察员”。过去的伤痛无法痊愈，被同事视为叛徒，孤独地处理着工作的每一天。在这种情况下，一封告密信传到了人事一课。搜查一课所属的原同事皆口菜子，非法泄露了机密数据。佐良一边对过去的伙伴清洗感到纠结，一边被上司能马命令监察，开始了大家的行动确认。于是，和某个未解决的杀人事件的接触点，以及斋藤殉职事件的真相浮出水面。背叛者是谁。密告文中隐藏的意图是。在疑惑和纠葛的尽头佐良好容易走到的冲击的真相是？

After a junior detective dies in the line of duty during an active investigation, Detective Masaki Sara of the Tokyo Metropolitan Police First Investigation Division is transferred to Section 1 of the Personnel Division, the internal affairs unit that investigates officers suspected of misconduct. Still traumatized by his past, Masaki is forced to carry out his duties as a lonely outcast as his colleagues shun him as a snitch.

One day, a message arrives. It's a tipoff that Saiko Minaguchi, Masaki's former colleague in the First Investigation Division, is leaking confidential information. Although Masaki is reluctant to investigate someone he used to work with, he obeys his boss and starts looking into Saiko. He soon discovers a link to an unresolved murder and some buried facts about the death of his junior detective Saito. What shocking truth awaits Masaki beyond his suspicions and inner conflict?

警視庁捜査一課の刑事、佐良正輝（松岡昌宏）はある殺人事件の捜査中に後輩刑事の斎藤康太（戸塚祥太（Ａ.Ｂ.Ｃ-Ｚ））を殉職で失い、警察職員の不正を取り締まる人事一課監察係に異動となる。過去の傷は癒えないまま、同僚からは裏切り者と敵視され、孤独に業務をこなす日々を送っていた。そんな中、一通の密告文が人事一課に届く。捜査一課所属時の佐良の元同僚、皆口菜子（泉里香）が、不正に機密データを漏らしているというのだ。佐良はかつての仲間を洗うことに葛藤を覚えながらも、上司の能馬（仲村トオル）から監察を命じられ、係長の須賀透（池田鉄洋）とともに皆口の行動確認を開始する。すると、ある未解決の殺人事件との接点、そして斎藤の殉職事件の真相が浮かび上がってくる。

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