Batman comes to New York City in his search for an arsenal of high-tech weapons and robots that use decapitated human heads as their brains. Antagonistic, confrontational and suspicious of each other, Spawn and Batman engage in violent battles before realizing they are both after the same villain. Grudgingly, they decide to work together. The person they seek has been kidnapping and decapitating the homeless for use in the robots, but that’s only part of the plan: There is also a nuclear arsenal ready to be deployed.
A note inside the front cover states that the one-shot is "a companion piece to DC Comics' The Dark Knight Returns. It does not represent current DC Comics continuity." Though the crossover has never been a part of the ongoing Spawn continuity either, it is a commonly held belief that the scar appearing on Spawn's face at the beginning of Spawn #21 is the same wound inflicted by Batman with a carefully thrown Batarang at the closing of the crossover, which is upheld by Spawn's remark in that very same issue that the scar was a result of his run in with "some bozo in black." However, through some slight retcon, specifically with the non-chronological release of issues Spawn #19 and #20, the "bozo in black" in question is actually revealed/changed to be Harry Houdini, and the scar in question is the result of gunfire that Spawn suffered while protecting Terry Fitzgerald.
^Manning, Matthew K.; Dolan, Hannah, ed. (2010). "1990s". DC Comics Year By Year A Visual Chronicle. Dorling Kindersley. p. 267. ISBN978-0-7566-6742-9. "This prestige one-shot marked Frank Miller's return to Batman, and was labeled as a companion piece to to his classic 1986 work Batman: The Dark Knight Returns. The issue was drawn by Todd McFarlane, one of the most popular artists in comic book history."