Two weeks following the events of Batman: Arkham City, several GCPD officers suddenly go missing. The culprit seems to be Harley Quinn, having escaped custody and undergone extreme mental stress following her recent loss. She has now taken over the industrial district of the old Arkham City site, which Batman infiltrates and ultimately gets himself captured so Robin will come to his rescue, and together, they can stop Harley and save the cops before the night is over.