It’s Goodbye Once a Flag Is Set Gintama : Season 4 Episode 37

Gintoki and Shinpachi start re-investigating the Rokkaku incident. Upon learning what really happened from Kamiyama, one of Okita’s subordinates, they leave to save Okita. Meanwhile, Okita is captured by Sotatsu Tendo after falling into his trap involving Kirie. Tendo babbles on about how he returned to seek revenge against Okita, who destroyed the Sokai Faction, and goes on to reveal the truth about Kirie’s father.