Set after the events of Arkham City, Harley Quinn is grief-stricken over the death of the Joker and has taken over the docks. After freeing a hostage, Batman is overwhelmed by Harley's crew and captured. After not hearing from him for a time, Oracle sends Robin to the docks to find Batman. After locating Batman's utility belt, Robin tracks down Batman, finding him in a sphere with only minutes of air. Robin tracks down Harley and steals her key-card, saving Batman before he dies. After being freed, Batman battles Wonder City robots and Titan henchmen before tracking down Harley's bombs and disabling them.