Doctor Who Series 10: 11 Easter Eggs & References You Might've Missed In 'Thin Ice'
6. Kill The Moon Callback
The Doctor and Bill decide that it all comes down to leaving the alien serpent where it is, where it is mistreated and trapped, or releasing it, even though it could kill thousands. The Doctor leaves the final decision to Bill. "It can't be up to me," he declares. "Your people, your planet."
This scenario is very similar to Kill The Moon, from Peter Capaldi's first series as the Doctor. In that episode, the Doc discovers that the moon is really an egg (just run with it) and that its hatching could cause untold destruction to the Earth below. So do they kill the moon or let it live?
The Time Lord has a much harsher method of getting his companion to make the decision on that occasion, though, as he leaves Clara in the lurch by disappearing in the TARDIS.