"Army of One" is a bonus story after the conclusion of Hellstar Remina. One of the most popular Junji Ito's one shot story, "Army of One" is unanimously praised by readers and is often said to be Ito's magnum opus - even as far as saying the short manga is better than it's parent manga, Hellstar Remina.
Michio is a hikikomori (a person who remains shut inside the house and withdraws from society.) He dropped out of school at age 13 and hasn't left the house for seven years. One day, an old classmate named Natsuko Horie stops by his house. She and some others are organizing a class reunion since this year everyone who went to middle school with them is "coming of age" - in Japan, people become legal adults on their 20th birthday. Natsuko was Michio's only real friend when he was at school. Because of his crush on her, he accepts her offer to meet some others at a cafe. Meanwhile, the dead bodies of a couple who went missing have turned up, naked and sewn together with fishing wire.
Michio goes with Natsuko and catches up with others he knew in middle school: Noriyuki Sakai, a popular classmate chosen to organize the reunion, and Keisuke Kasagi. However, when Michio discovers that Natsuko is engaged to marry Sakai that year, he gets dispirited and leaves. On the way home he comes across a scene where six bodies have been found sewn together. A plane flies overhead and drops fliers: "Nobody likes a lonely only. Everybody join hands now. Everybody join hearts now." These are the words of a commercial that has been playing on the radio, encouraging people to join the "Army of One", but providing no further details.
On the internet, Michio chats to other hikikomori and learns of rumors that the Army of One is behind the so-called "stitch murders." He is interrupted when Natsuko calls, asking if he has seen Keisuke. Keisuke went to a lonely hearts party and never returned (having invited Michio, who turned it down.) His body is found on a mountainside several days later, sewn together with those of the other party guests.
Michio is told by others online that the "Army of One" will target large groups. He tries to warn Natsuko to cancel the reunion but she refuses, as she believes Keisuke would want them to hold it. On Christmas Eve, the "Army of One" song plays over the radio and the following day, hundreds of dead bodies turn up, sewn together into bizarre patterns and decorating the streets and Christmas trees. The Army of One is subsequently dubbed a terrorist group, with people warned not to congregate in groups. No one is sure what the Army of One could really be, with some speculating they are an alien presence.
The class reunion and "coming of age" ceremony isn't canceled. Michio tries to sneak in to see if Natsuko is safe, but is caught by security guards and mistaken for an Army of One spy. Natsuko finds him and persuades the guards to let him in, but when she and Michio arrive, all five hundred people who had been waiting for the ceremony are missing. They later turn up sewn together, including Noriyuki.
From then on, people stop going outdoors. Michio calls Natsuko, who is depressed over Noriyuki's death and becoming suicidal. Michio is prepared to risk his life and go to her house to keep her company. While he is outdoors, the "Army of One" planes fly overhead and drop fliers; but the Japanese air force arrives to do aerial battle with them. Michio, heartened by the prospect of the Army of One's defeat, runs over to Natsuko's house to tell her he loves her. He finds her sewing herself to her (dead) parents and their dog, singing the Army of One song to herself, much to the horror of Michio who watches this. Overhead, the military planes have somehow begun dropping the Army of One's fliers as well.