The Doctor invites Bill to take a trip with him in the TARDIS, ignoring Nardole's objection that the Doctor swore an oath to not leave Earth except in cases of emergency.
She asks to go to the future, so the Doctor lands the TARDIS in a field of a human colony settled after Earth was evacuated. Strangely, there are no people to be found in the whole colony, only emoji-speaking robots.
Actually, the "robots" are more user interfaces for swarms of microbots that actually form the physical structure of the colony itself. As the Doctor and Bill explore the colony, they realize that the bots actually killed the inhabitants and turned them into fertilizer.
The Doctor realizes that the unfortunate humans were merely the forerunners of the actual colonists, and that countless more innocent humans would fall victim to the same trap, so he decides to destroy the colony.
Before he can do so, however, Bill discovers that the colonists are already there; they were merely in suspended animation, and they're now waking up.
They realize that the bots had achieved sentience, but in their desire to help humans did not understand the concept of grief. When one of the original expedition members died of natural causes, the reactions of the others caused the entire tragic affair.
The Doctor manages to reset the bots' protocols, but warns the now-awake humans that they will need to find a way to coexist peacefully with their new neighbors.