PBS has released a clip from this Sunday’s Sherlock season 3 episode, titled “The Sign of Three.”

In terms of where this scene appears in the 90-minute episode, this Sherlock season 3 clip starts as a flash backward just as the lead detective begins giving his best man speech at John and Mary’s wedding reception. We also see Mrs. Hudson having a laugh over a telegram (which disturbs Watson), and Molly and Greg discussing Sherlock pulling off a best man speech. Towards the end of the clip we see Sherlock relaying messages from guests who couldn’t attend the special day.

Much of the wedding is a frame for several flashbacks which will show off Sherlock and Watson’s friendship over the years, including some great sequences involving a night out and Watson asking Sherlock about being his best man.

Naturally, some trouble does crop up at the wedding which you’ll see when Sherlock season 3, episode 2, “The Sign of Three,” airs this Sunday at 10 p.m. eastern/pacific on PBS (following a new episode of Downton Abbey).

“The Sign of Three” is the second of three episodes in the third season of the British detective drama starring Benedict Cumberbatch and Martin Freeman. The new episodes have already aired in the United Kingdom earlier this month.

The synopsis for episode two reads, “Sherlock faces his biggest challenge of all – delivering a Best Man’s speech on John’s wedding day! But all isn’t quite as it seems. Mortal danger stalks the reception – and someone might not make it to the happy couple’s first dance. Sherlock must thank the bridesmaids, solve the case and stop a killer.”