This season continues the plot of "Sherlock Holmes Season 2". Two years after the Reichenbach fall, Watson (Martin Freeman) strives to get his life back on track. Mary Morstan (Amanda Abbington), who combines beauty and wisdom, will enter a new stage of harmonious and romantic life with Watson. However, at this moment, Sherlock (Benedict Cumberbatch) staged a resurrection drama. Although this is Watson's long-praised wish, when the wish comes true, he still can't face everything calmly. This time, Sherlock and Watson need to solve the mysterious conspiracy under the streets of London, the sudden accident at the wedding, and face the notorious and insidious king of blackmail-Charles Augustus Magnussen (that is, Milverton in the original book, played by Lars Mikkelsen). Sherlock is back, but is everything the same as before?