This issue, Mage: The Hero Defined #0, is titled the "Second Interlude." It takes place after the series Mage: The Hero Discovered, and also after the "First Interlude," which was published inGrendel #16-19. It takes place before Mage: The Hero Defined #1.
Kevin Matchstick has continued to gain experience as a warrior against the forces of darkness. Here, he has a more carefree attitude than in the first Interlude. He is searching the Grand Canyon for some "nasties" and runs into Joe Phat, a fellow avatar. Joe and Kevin fight an ogre-wight and destroy it through teamwork. They bond quickly and decide to join together in their struggle against their eldritch adversaries. This story sets up the partnership that they continue throughout the rest of The Hero Defined proper.
It is not necessary to read the Second Interlude in order to understand the rest of The Hero Defined, in terms of plot.