Sherlock Season 4: 20 Easter Eggs & References From The Six Thatchers

7. Agra

Back in His Last Vow, when we found out that Mary Watson was actually an ex-assassin, we also learned that everything about her dark past was contained on a memory stick marked 'A.G.R.A.' This was a reference to the treasure of Agra, the maguffin of The Sign of Four - Mary's debut story in the canon.

In The Six Thatchers, it turns out that A.G.R.A. stands for the initials of her squad of secret agents (Mary's real name is Rosamund). When Sherlock asks him what Agra means, Mycroft sarcastically comments that it is a city in India. This is what it refers to in The Sign of Four.