Having travelled back in time to Victorian-era New York City to save her siblings, Constance Greene confronts Manhattan's most dangerous serial killer, Dr. Leng, but she is betrayed and left incandescent with rage.

Agent Pendergast is determined to assist Constance, who is now on a fanatical quest for vengeance. And Diogenes, Pendergast's brother, also takes a trip out of his own time, offering to help the duo for unexplained reasons of his own.

Diogenes establishes himself in New York's notorious Five Points slum, watching Dr. Leng's every move and awaiting a chance to strike. But Constance strikes out on her own, intent on savage retribution.

Yet Dr. Leng is one step ahead of everyone, and still has a macabre surprise in store...