 Major concepts of the finale were already developed in March 2007 . Davies explained the Medusa Cascade — first mentioned in dialogue between the Master and the Doctor in " Last of the Time Lords " — to Radio Times and Doctor Who Magazine journalist Benjamin Cook as " just an area of space " near an inter @-@ universal rift which allowed Rose to return for the fourth series . He sent Cook another email several hours later that explained Dalek Caan 's role in the finale and Davros ' resurrection from the Time War . The Doctor 's regeneration was conceived in two separate parts in mid @-@ 2007 : Davies outlined the concept of two Doctors in " Journey 's End " in late April 2007 ; and using a regeneration to end the episode was originally conceived on 12 July 2007 . 
