It is John Watson and Mary's wedding day. Holmes is the best man and has laboured over his speech, plus micromanaging the reception. While giving his speech he recounts how Watson invited him to be best man, the wedding plans and recent cases of theirs. One of the cases involved a member of the Household Guards who believed he was being stalked, and then was found stabbed in the shower, with no weapon in sight. Another case seemed bizarre - a woman who believed she had had a relationship with a ghost.
Though this episode does involve crimesolving, the focus is really on Dr Watson's wedding. This becomes a great opportunity to demonstrate Holmes' social awkwardness and the many comedic possibilities this presents. Much amusement ensues. It also provides many emotional scenes and Holmes proves himself to be unconsciously reasonably sentimental.
The crime solving is quite intriguing too, tying three seemingly independent events together.