When Spike sees a bounty for someone named Mao Yenrai, he decides to pursue it alone without telling Jet why. Turns out Mao was Spike's mentor back in the Red Dragon Syndicate, but now someone's killed him—Vicious, Spike's former partner. Through flashbacks we learn that Spike used to be a syndicate assassin who tried to leave the organization with a woman named Julia, who was Vicious's woman. Meanwhile, Faye gets captured and used as bait to lure Spike into the trap.
The whole thing leads to an abandoned cathedral where Vicious is waiting with his katana and a bunch of crows. Their confrontation gets brutal and personal, with Vicious calling out Spike for his betrayal. The fight ends with Spike falling through a massive stained glass window in slow motion while "The Real Folk Blues" plays. Jet finds him later, badly wounded but alive. The episode basically changes everything—now we know Spike has a past that's going to catch up with him eventually.
This is the series' first major plot episode and introduces Vicious, the main antagonist. The cathedral scene with Spike falling through the stained glass window became one of anime's most iconic images. "Green Bird" by Yoko Kanno plays during the confrontation.