The Avengers S04 E05 Castle De'ath

"Mrs Peel, We're Needed!"

Why have all the fish vanished from the Scottish coastline? Does it have anything to do with a dead frogman, found stretched as if on a rack? Steed and Emma think so, which is why they're guests of Ian, the 35th Laird of Clan De'ath, and in danger of being caught by the gillies.