Doctor Who - 10x1

Bill becomes fascinated by a girl named Heather, who has a distinctive "star in her eye." They find a puddle that doesn't reflect correctly; it is actually a sentient fluid from an alien ship.

Bill is drawn to the Doctor’s lectures and he eventually agrees to tutor her. Doctor Who 10x1

The fluid mimics Heather and pursues Bill and the Doctor across time and space—including a brief encounter with a Dalek battle—to find a "pilot". Bill becomes fascinated by a girl named Heather,

This episode sets up the season-long mystery of the "Vault" beneath the university, which the Doctor and Nardole are sworn to protect. Doctor Who 10x1

It was designed as an entry point for new viewers, explaining the TARDIS and the Doctor’s nature through Bill's inquisitive eyes.