Sentient oil
A puddle of sentient oil was left behind from a spaceship that had landed in Bristol before later taking off again. The oil was able to mimic other beings, survive a blast from a Dalek gun, track beings and and following them around across time and space.
Heather had previously come across the puddle around St Luke's University, noticing something strange in her reflection. She took Bill with her to show her the puddle, who also noticed something strange in her reflection; however, Heather was pulled into the puddle and became its "pilot", as she wanted to leave.
Bill told the Twelfth Doctor about the puddle. Before Bill finished explaining, the Doctor had already run to the site. The Doctor quickly discovered what was off about the reflection, which was that it was showing the reflection in the other way to a mirror and everything was in the right place. The Doctor also discovered scorch marks in a circular pattern surrounding the puddle, but left claiming he was no longer interested. As they left, the oil started creeping away.
Bill later returned to St Luke's only to see Heather there again, only drenched in the oil and repeating everything Bill was saying. Bill ran to the Doctor's office and shut the door behind her, locking it in place with a chair. However, the puddle managed to slide through the crack underneath the door and appear as Heather again. To test whether the "alien menace" was after them or what was in his vault, he first took his TARDIS to the vault where Nardole was. After he realised Heather was after them they ran to the TARDIS and landed in Sydney. The Heather Creature appeared there too.
Determined to find the limitations of the oil, the Doctor travelled 23 millions years into the future onto another planet. The oil followed them through time to this planet as well. The Doctor and Bill went back into the TARDIS and landed in the middle of a Dalek-Movellan War to get a Dalek to exterminate the oil.
Upon doing so, the oil survived the attack, saying the Dalek's "exterminate" phrase whilst doing so. The Doctor and Heather saw a Dalek, but realised it was the oil, having taken the form of a Dalek with Heather's star eye. It transformed again into Heather. The Doctor realised that the oil didn't want to kill, but wanted Bill. Bill told him that Heather promised she wouldn't leave her which was why the oil kept following her. Bill told the oil to let her go, and the puddle disappeared. (TV: The Pilot)
