Naoto Kuniyoshi is a bored, misanthropic youth who feels he was born in the wrong era, believing that he has more in common with the samurai and ninja of Japan's past than those living in the decadence of the present day. Having been nick-named "Ghost Boy" for his ability to go through life almost completely unnoticed by those around him, the twenty year-old student spends his days setting up spying equipment to stealthily take pictures of young women around his college campus, which he later photoshops into pornographic images for his "collection."
Everything changes when Naoto is recruited by an enigmatic coordinator to participate in a special "tournament" with promises that he will lead the life of a billionaire should he win. The catch: to progress through each of tournament's 8 rounds, Naoto must find a way to kill his opponent using only indirect means, with all of Japan designated as their arena. After unwittingly agreeing to join the tournament, Naoto is immediately thrust into a world of murder, torture, and espionage that he never dreamed could exist in the modern day.
Underdog includes examples of the following:
- Body Paint: Hiuchi is introduced wearing this, and wears it when he meets Naoto.
- Bound and Gagged: Multiple characters so far, this tournament being what it is.
- Deadly Game: The year-long tournament, wherein the goal is to be the sole survivor of the 200 participants that initially entered. In each round, a player wins by killing their opponent through indirect means only (meaning that they can't directly injure their opponent, confine them in a way that causes their death, or hire others to kill them.) Otherwise, anything goes.
- Evil Laugh: Hiuchi does this.
- Frameup: Hiuchi's first action in the tournament is to steal Naoto's wallet so that he can plant it on the body of a high school girl Hiuchi just murdered. He then breaks into Naoto's house to place her student ID on his desk. And later, where he frames Naoto for the assault of his own sister.
- Hoist by His Own Petard: Ryouko abuses her power to allow Hiuchi to sneak out of the mental ward allowing him to commit crimes. She becomes his final victim after he is shot up and falls onto her over a tall building.
- Left Hanging: The tournament was suppose to run for at least a year but chronologically Naoto only participated for a total of a month. The manga was cut short for reasons unknown.
- Let's Get Dangerous!: Naoto is fully aware of his situation, notably by the tenth day, where he has Ryouko Onogi Bound and Gagged to give him details on Masaya Hiuchi, right after he takes a snapshot of her sexually taking advantage of a younger patient, and proceeds to chase after Hiuchi to enact his plan, leaving his Mirai in Yuuki's care.
- The Peeping Tom: Naoto, before entering the tournament. This is a trait that is used against him by his first opponent.
- Serial Killer: Naoto's first opponent, Masaya Hiuchi, is an 18 year-old who was just recently released from a mental institution after brutally murdering several of his female classmates in middle school.
- Slasher Smile: Fittingly, Hiuchi is seen wearing this when he first introduces himself to Naoto.
- Say It with Hearts: Noa Takayanagi is often seen doing this.
- Take Over the World: What Noa introduces the goal of the game as to Naoto.
- Tears of Fear: Early on, when Naoto is trying to find his sister, kidnapped by Hiuchi.
- Teens Are Monsters: Or enough are from Hiuchi's perspective to be convinced of that, which would explain how violently Hiuchi reacted upon Mirai saying she was jealous of how thin he is, and somewhat explain how he got to be like he is.
- The Vamp: The buxom tournament coordinator Noa Takayanagi, who uses her feminine wiles early in the series both to convince Naoto to participate in the tournament, and to get him out of trouble with a couple of police officers on patrol by distracting them with her cleavage.
- Would Hurt a Child: Early on Naoto pours Tabasco sauce on a disruptive child.