Steven Moffat

Bill meets a mysterious student named Heather who is fixated on a strange puddle that hasn't dried up despite a lack of rain. As they investigate the reflection, an unsettling realization begins to dawn on Bill, while Heather abruptly flees in fear.

The Doctor summons Bill Potts, a canteen worker who has been sneakily attending his university lectures, to his office. He challenges her on why she attends and ultimately offers to become her personal tutor, despite her not being an enrolled student.

The Doctor and Bill investigate a mysterious puddle that doesn't reflect, but mimics. The Doctor realizes the water is actually an alien entity before abruptly ending the investigation to protect Bill.