Rumor is the prologue to the fourth Durarara!! light novel.
A Russian assassin called the "Murder-Machine Philosopher" and the serial killer Hollywood cross paths in Ikebukuro and begin to fight. During the fight, the assassin—who is known for using whatever he can get his hands on—snatches a bystander's briefcase to use as a weapon. The briefcase belongs to Tom Tanaka; assuming that the assassin is stealing the briefcase, Shizuo Heiwajima knocks the combatants out with a park bench.