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)

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.