Densha Otoko (電車男), aka Train Man, by Hitori Nakano, 2004.

Based on the purportedly true story of a 23-year-old otaku who intervened when a drunk man started to harass several women on a train. The event was chronicled on 2chan. This led to the compilation of the relevant threads in Densha Otoko.

(Link added in 2024)