Daleks' master plan

Centuries before the Twelfth Doctor's regeneration, the Daleks lost their copy of the Daleks' master plan, recorded on a VHS tape.

The Doctor, on the other hand, kept a copy, (PROSE: Twice Upon a Time) 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)

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.