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. In the middle of the fight the assassin known for using whatever he can get his hands on takes a bystander's briefcase to attack Hollywood. The briefcase belongs to Tom Tanaka, and assuming that it is being stolen, Shizuo Heiwajima knocks the two out with a park bench.