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
Rando uses his ability to create a certain space under his control outside the normal laws of physics, to prevent Chūya's gravity from having an effect, and also reveals that he can reanimate corpses and store them for use as fighting slaves; having done so with the Predecessor's and uses it to fight Dazai. Dazai and Chūya work together and beat Rando, who reveals he used to be a Foreign spy. He came to Japan eight years ago, with his partner to inspect the reported containment of a powerful entity by the Japanese government, however his partner betrayed him which resulted in a fight and the partner's death. When the military was alerted of the battle, Rando was unable to fight back and attempted to reach out and store Arahabaki from its separate space as his weapon. The attempt failed and resulted in the explosion that formed Suribachi city and Chūya, who he claims is not Arahabaki, but merely a human vessel. Rando passes away, and when Chūya later goes to his grave, he is confronted and wounded by the Sheep who try to kill him; The Sheep fear the possible day when Chūya could side with the Mafia and have partnered with GSS to kill him. Chūya escapes, and Dazai approaches him with an offer from Mori to join them. Chūya accepts, demanding that they do not kill the Sheep which Dazai accepts. Chūya is given a hat from Rando's possessions (It is a ritual in the Port Mafia for a junior to receive a belonging from the senior who invited them) swears loyalty to Mori, and seven years later, Ango is making a note of these events and mentions that Rando's partner was still alive at the time, and Chūya would later confront them.