The Hornets were an insect species which resembled the hornets of Earth. They came to Earth to invade. They possessed stuffed animals and Percy Noggins, the director of a factory. They thought the Fourth Doctor would be a good host, but he avoided this by escorting them to his home at Nest Cottage. (AUDIO: The Stuff of Nightmares)
By stinging famous dancer Ernestina Stott, the Hornets were able to possess her and used Mrs Wibbsey's voice to command her to sacrifice her life to the sea. They possessed a China Doll in order to speak with the Doctor while he was inside the Victorian house. (AUDIO: The Dead Shoes)
The Hornets then were hunted by the Doctor through time and space until he came to a circus called the Circus of Delights. The ringleader was possessed by Hornets, and the plot was to kill the town by enlisting them in the dreadful "Circus of Delights" where... accidents happened. They would have possessed/killed the entire town had the Doctor not stopped them.