 Two horses were used in the episode ; one was used for the scenes in close quarters on the spaceship , and another for jumps . According to Doctor Who Confidential , the horse was not allowed into the ballroom for the climactic scene . Thus , the elements of the Doctor riding through the mirror , the horse , the mirror breaking and the reactions of the extras in the ballroom had to be filmed separately and then composited together with chroma key . Tennant 's head was superimposed upon that of the stunt rider in post @-@ production . Initially , the programme 's staff considered the use of special effects but realised this would be very expensive and rejected the idea . 
