The Hero Defined begins many years after The Hero Discovered. Kevin Matchstick has moved past his nascent stage and has become defined as a hero. He is more confident in his abilities and demonstrates greater control over them. Mirth disappeared long ago, and he is now working with Joe Phat, another hero who is the reincarnated version of the Native America Coyote hero. Using Joe's ability to smell magic, the duo follow Joe's nose to a urban area to fight a Bridgewight troll. The next day as they're leaving Joe sniffs out another creature and they find a nest of harpies. Here they meet Kirby Hero, known as the Olympian, and the reincarnation of Hercules. Kirby has been charged with completing twelve labors by his father, and the harpies are part of his quest. The harpies are quickly defeated and Kirby agrees to join Joe and Kevin for a while. During their conversation, Kevin mentions how Mirth left him and explains that he's waiting for his second mage. The old man who the group rescued from the harpies' control introduces himself as Wally Ut, and declares that he is Kevin's second mage. Kevin dismisses the old man's ranting and the new group of Kevin, Kirby, and Joe head out.
The trio are drawn northwards by an unknown compulsion, and Kevin brings them to the home of Isis Hunter and Bartholomew Gretch. Isis uses her magic to lead them to a vision of the power behind the forces of evil. It shows them a large creature on a mountain. They are then attacked by Sprigginflints, the sons of Emil Grackleflint, now known as the Pale Incanter. They force the Sprigginflints to retreat and drive up to Montreal, where they feel the evil is amassing. Unknown to them, Emil is siphoning off part of Kevin's power every time he uses it, and is redirecting that power into a giant golem.
When they get to Montreal, they find themselves in several battles with evil creatures. They also find that several of their fellow avatars have come to the city, but unfortunately they are fighting amongst themselves as much as they fight the enemy. Kevin tries to establish some leadership among the group, but many of them prefer to be independent. In fact, even Kirby, one of Kevin's closest allies, is unhappy with Kevin's attempts to be the leader. One of the few who unquestionably follows Kevin, the Hornblower, is kind of annoying, so Kevin brusquely sends him off on a wild goose chase, where he is killed. Kevin has come to a point where he is simultaneously feeling guilty over everything he does, and feeling totally cocky and arrogant about who he is, and those two traits are working against him. He fights his fellow avatar the Dragonslayer against his better judgement, and while he beats him, his Bat (a disguised Excalibur) burns his hands for using it for the wrong reasons.
There are further complications as Wally Ut shows back up, still proclaiming his status as the second Mage and giving insane-sounding advice, and as Isis comes to visit along with her sisters Magda and Ishtar. Kevin is said to be fated to fall in love with Magda, a fate he had tried to avoid. However, he instantly falls for her when he sees her. It seems to feeling is mutual. However, it is unclear whether it is true love or a magic enchantment, for he frequently seems to be completely hypnotized at the thought of her. Also, Joe is secretly ensnared by a Leanhaun Sidhe, a succubus, who takes over his mind and drains his energy, unknown to the others. Finally, Kirby keeps leaving and coming back unexpectedly as he tries to complete his own set of labors.
Slowly the heroes manage to find out where the source of evil is coming from--the giant cross on Mount Royal. They also learn that Emil is behind it all. The succubus kidnaps Joe, and Kirby, Kevin, and their new ally the Presbyter go after him, up at the cross. They find that the cross hides a passageway down into Emil's faerie realms. The Presbyter is soon lost, but the heroes rescue Joe and even find Wally Ut. They finally get to an arena where Emil and his golem are waiting. Kirby thinks that killing the golem is one of his labors and is angry that Kevin won't back off. Eventually due to a mistake of Kevin's, the golem gets even more power and kills Kirby. Kevin manages to resurrect him but destroys the Bat in the process. Wally Ut, meanwhile, turns out to actually be the second mage after all, and defeats Emil. Emil is tricky, however, and escapes, only to be banished by his father, the Umbra Sprite. Kevin and Wally run out, but Wally is lost. At the end, Kevin finds Magda and asks her to marry him.
Artistically, the Hero Defined is quite different from The Hero Discovered in many ways. The Hero Discovered uses watercolor and airbrush for colors, along with a very precise ink line and large solid black areas. It has a very graphic look. The Hero Defined drawn in a much looser, sketchier, more cartooney way. Its colors are done in a more standard, early-computer-coloring style.
In terms of writing, one major difference is that while The Hero Discovered was semi-autobiographical, with Kevin Matchstick based on Matt Wagner, the rest of the characters seem to be purely fictional. In The Hero Defined, on the other hand, many of the characters are explicitly based on Wagner's acquaintances. These include: Joe Phat is Joe Matt, Kirby Hero is Bernie Mireault, The Dragonslayer is Dave Sim, the Ulster Hound is Alan Moore, Isis is Diana Schutz, Bartholomew Gretch is Bob Schreck, John J. Strider the Xth is John K. Snyder the III, and Magda is Wagner's real wife, Barbara Schutz (sister of Diana).