A Sting In The Tale is the fourth and penultimate chapter in the Hornets' Nest story with the Fourth Doctor and Captain Yates.
Included in the cast here is Rula Lenska (Styles in Resurrection of the Daleks) playing the Swarm.
The Doctor recounts to Mike Yates of tracking down the earliest manifestation of the Hornets at a nunnery in the 11th century.
The pieces are falling neatly into place.
I found A Sting In The Tale to be most thrilling during the second half including when the Doctor encounters the Swarm.
A very good cliffhanger leading to the final chapter of the tale Hive of Horror.