Episode 28

Only a Diamond Can Polish a Diamond (ダイヤはダイヤでしか,) is the 28th episode of Bungo Stray Dogs anime series. It first aired on April 26, 2019.

Synopsis
Chūya and Dazai reveal the truth behind Arahabaki, Rando, the former Port Mafia boss, and Chūya himself.

Summary
Chūya reveals a little about his past and interrogates Rimbaud where did he find him - grabbing Chūya to the outer world. Rimbaud attacks Chūya with his ability, eventually enclosing the three of them, including Dazai, in a closed space. Chūya tries to attack Rimbaud but fails to do so. In response, Rimbaud continuously attacks Chūya with his hyperspaces.

Out of nowhere, Dazai is attacked by the former Port Mafia boss. Dazai asks Rimbaud about his ability, to which Rimbaud openly tells them. Rimbaud reveals his past nature as a former western spy and his order to obtain the Arahabaki - particularly killing Chūya and subsuming him as a replacement for the former Port Mafia boss.

Dazai proposes an idea to Chūya to kill both Rimbaud and the former Port Mafia boss together. The former Port Mafia boss attacks Dazai. Meanwhile, Chūya engages in a battle with Rimbaud. Dazai and Chūya combine their power to let Dazai nullify Rimbaud's ability - eventually also killing the former Port Mafia boss.

After the battle, Rimbaud reveals that he and his partner have come to Yokohama to obtain the Arahabaki eight years ago - which results in them being arrested and eventually unleashing Arahabaki to escape from the facility, but unfortunately, it caused the explosion of the building - resulting to Rimbaud losing his memories. Rimbaud tells his thoughts about Chūya being only a vessel to Arahabaki. Rimbaud soon after dies.

Chūya is seen near the grave of Rimbaud. After leaving, he encounters Shirase - eventually taking the opportunity to stab Chūya while they are talking. The Sheep reveals themselves to have teamed up with GSS to fight against Port Mafia. Chūya manages to escape from them by using his ability.

Dazai, accompanied by Port Mafia guards, calls out to him and relays a message from Mori to officially recruit Chūya to the Port Mafia. Dazai reveals that the Port Mafia plans to kill GSS and Sheep. Chūya proposes a deal not to kill the children, so Dazai orders the Port Mafia guards to do so.

At the Port Mafia base, Mori gives Chūya Rimbaud's hat. After much talk and learning what a leader is from Mori, Chūya bows and gives his dedication to him - eventually working for the Port Mafia. Mori welcomes him to the organization.

Kōyō is seen accompanying Chūya, while Mori is seen with Dazai and Q. Dazai and Chūya continue to bicker after encountering each other. Kōyō asks Mori if it will be okay to have the two in the organization, to which Mori replies that only a diamond can polish a diamond, words spoken by Natsume to him and Fukuzawa.

Seven years later, Ango is writing a report about the recruitment of Chūya to the Port Mafia and the truth of Chūya's emergence one year later - confronting Rimbaud's friend and betrayer.

Abilities

 * Illuminations
 * Upon the Tainted Sorrow
 * No Longer Human

Trivia

 * Setsuna no Ai was used as an insert song in this episode.