Gardner considered this scene untruthful and too confusing, since Donna remembering would lead to her death, and since she didn't recognise the Doctor it wouldn't make sense to assume she would recognise the noise of the TARDIS. This episode's original ending involved the Doctor, following the final scene where he is alone in the TARDIS, being alerted to something on the monitor and as he checks two Cybermen rise up behind him. This was supposed to lead directly into "The Next Doctor", but Davies was convinced by Benjamin Cook (who was corresponding with Davies for the book "Doctor Who: The Writer's Tale") to drop the scene, as he felt that a cliffhanger was not appropriate after such a sad ending. Castell Coch, situated minutes away from "Doctor Who"'s Upper Boat Studios, is used as the German castle. The beach at Southerndown, a few miles west of Cardiff, is used once more as Norway's fictional Dårlig Ulv Stranden (Bad Wolf Bay). Some exterior scenes, including various companions interacting with Daleks, were shot at Arcot Street, Penarth. Mickey Smith and Jackie Tyler make their first appearances in "Doctor Who" since "Doomsday". K9 Mark IV (voiced by John Leeson) makes his first appearance since "The Sarah Jane Adventures" story "The Lost Boy", and his first in "Doctor Who" since "School Reunion". Former "Blue Peter" presenter Gethin Jones controlled one of the Daleks that escorts the human prisoners aboard the Crucible. He previously played a Cyberman in "Rise of the Cybermen" and has made a cameo appearance as himself in "Doctor Who" spin-off "The Sarah Jane Adventures" in the episode "Invasion of the Bane". One significant feature of this episode is the creation of another Doctor. Unlike the multiple Doctors of stories such as "The Three Doctors", "The Five Doctors" and "The Two Doctors", where his previous incarnations were played by actors or depicted in old footage, this Doctor is identical in appearance to the Tenth Doctor. In the accompanying "Doctor Who Confidential" for this episode, Davies explains "This is so busy and so "mental" and so epic and universal in scale that of course you need two Doctors to solve it." Phil Collinson, Graeme Harper, and David Tennant discuss the use of the double, a musician named Colum Regan who is a very good physical match for Tennant. Collinson explains that while with an unlimited budget they would use Tennant in every shot, "we only have a certain number of effects shots where you can see the two Doctors together, so we have to pick those carefully."