Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'

A much more overt reference to another British institution can be found in the notion that Eurus appears to be an inmate in her secure island prison but is actually in charge of the governor.

This is exactly what the Master does in a Doctor Who story from 1973 titled The Sea Devils. In that, the Third Doctor visits his old enemy in jail and it appears that everything is in order. It only transpires later that the Master has used his hypnotic control to put the governor under his thumb.

As well as borrowing from the Master, Eurus is also partly inspired by Hannibal Lecter, Anthony Hopkins' infamous cannibal. This is referenced in the episode itself when a guard explains to Sherlock: "It's basically The Silence of the Lambs."