Daleks' master plan

Centuries before the Twelfth Doctor's regeneration, the Daleks lost their copy of the Daleks' master plan.

The Doctor, on the other hand, kept a copy that had been recorded on a VHS tape. (PROSE: Twice Upon a Time) It was later found by Archibald Hamish Lethbridge-Stewart, much to his confusion, as he rifled through the — to him, inexplicable — decor of the control room. (TV: Twice Upon a Time)

The eponymous master plan referred to the Daleks' attempt to develop the Time Destructor on Kembel, which they and the Doctor thought of as "the Daleks' masterplan". (PROSE: Mission to the Unknown, The Mutation of Time, The Evil of the Daleks) Indeed, during the Time War, the Time Lords were aware that the Supreme Dalek who pursued the First Doctor through time went on to oversee the master plan. (PROSE: Dalek Combat Training Manual)

Human historians in the post-Time War universe understood that the Dalek Supreme was the last of the Dalek Supreme Council by the dawn of the 40th century. Having observed the failed stratagems of its rivals and learned from their mistakes, the Dalek Supreme set in motion the plan that would draw on all the lessons of Dalek history, a plan that almost brought the universe to its knees, taking the better part of a century to enact. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Behind the scenes

 * The idea of a lost "Daleks' master plan" taking the form of a VHS tape is an allusion to the missing episodes of the real-life TV serial The Daleks' Master Plan: Paul Cornell cheekily presents the Doctor as hoarding legendary Doctor Who missing episodes within the TARDIS.