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

John's late wife Mary has one more trick up her sleeve when she posthumously sends Sherlock and John a second DVD message. This one tells them to rebuild their lives and to be the defenders of the desperate forever. She calls them "the best and wisest men she has ever known."

This is a callback to what John called Sherlock in The Empty Hearse when they were trapped in a train carriage that was apparently rigged to explode. That instance was itself also a callback to what Dr Watson says of Holmes in Doyle's The Final Problem, when he thought his friend had fallen into the Reichenbach Falls.