In the days when an arc would probably refer to a floating cryogenic repository, Doctor Who committed itself to a lengthy 26-episode run of intertwined stories. The Key To Time ties Season 16 together very simply with the concept of a quest. Basically, the Doctor has to hunt through time and space for six separate parts of a key (not an actual key you understand, it’s a floating plastic cube) that when linked, will put the universe in balance and end the threat of eternal chaos.
As tasks go, this is something pretty daunting – this isn’t your archetypal Apprentice task in which SirLordWhateverHisBleedinTitleIsAlanSugar sends a horde of sweaty wannabes to hawk for business at a local art gallery. No, this is huge. For such an important mission, the Doctor gets an oddly low-key briefing by the awesome White Guardian.