https://www.wikiwand.com/en/Doctor_Who:_The_Key_to_Time

The Key to Time is the umbrella title that links all six serials of the sixteenth season of Doctor Who. The arc was originally conceived by producer Graham Williams, who had proposed it as part of his application for the producer's job in 1976.[1] The name refers to the powerful artefact for whose segments the Doctor, Romana and K-9 are searching during the season