10 Even MORE Doctor Who Episodes You Didn't Know Were Connected

Do you have recurring nightmares? The Doctor certainly does, if Robot and Listen are to be believed.

Listen centres on the Twelfth Doctor exploring the notion of fear itself. From a children's home to the end of the universe, the Doctor is determined to understand the monsters that might be lurking under the bed.

But it's not the monsters under the bed that appear to be playing on the Doctor's mind at the end of the episode –  it's the Sontarans! After being knocked unconscious, the Doctor eventually bursts awake with the words:

"Sontarans! Perverting the course of human history!"

This is obviously a nod to the Third Doctor serial The Time Warrior, in which both the potato-heads and Sarah Jane Smith first appeared. But it's also a more direct reference to the Fourth Doctor's debut story, Robot.

That's because these were the first words spoken by the Fourth Doctor after his regeneration. It's easy to miss Tom Baker's muttered words, but Sarah Jane clarifies them for the Brigadier and the audience, explaining that he's mumbling about something that happened when they first met.

Given that Listen is about constant companions, be they our fears or our friends, it's appropriate that Steven Moffat chose to reference The Time Warrior. It's very touching that the Doctor and Sarah Jane's first meeting is still at the front of the Twelfth Doctor's mind so many centuries later.