A trail of corpses sends the Dark Knight in pursuit of the serial killer known as the Sleeper...who is committing murder in order to save her own life.
Batman investigated a series of murders in which the victim ended up without its heart, but it appeared they stopped beating before foul play was involved. He found a pattern in the victims and awaited the killer at the next scene. He wasn't quick enough to save the target, and he ingested some of the heart-stopping gas. Hallucinations and panic followed.