Why Did the Siege Happen?

Mitsuhide Nira, who has been revealed to be the murderer of Eisuke Tomari, takes steps with Brain to hide away the evidence from 12 years ago. Moreover, they hold a press conference, falsely accusing the Special Investigation Unit of colluding with the Roidmudes, landing them in jail and Shinnosuke as the public's most wanted criminal. Shinnosuke must find a way to free the Unit and expose Nira of his crimes.