A criminologist narrates the tale of the newly engaged couple Brad Majors and Janet Weiss who find themselves lost and with a flat tire on a cold and rainy late November evening, somewhere near Denton, Ohio. Seeking a telephone, the couple walk to a nearby castle where they discover a group of strange and outlandish people who are holding an Annual Transylvanian Convention. They are soon swept into the world of Dr. Frank N. Furter, a self-proclaimed "sweet transvestite from Transsexual, Transylvania". The ensemble of convention attendees also includes servants Riff Raff, his sister Magenta, and a groupie named Columbia.
In his lab, Frank claims to have discovered the "secret to life itself". His creation, Rocky, is brought to life. The ensuing celebration is soon interrupted by Eddie (an ex-delivery boy, both Frank and Columbia's ex-lover, as well as partial brain donor to Rocky) who rides out of a deep freeze on a motorcycle. In a jealous rage, Frank corners him and kills him with an ice axe. He then departs with Rocky to a bridal suite.
Brad and Janet are shown to separate bedrooms where each is visited and seduced by Frank, who poses as Brad (when visiting Janet) and then as Janet (when visiting Brad). Janet, upset and emotional, wanders off to look for Brad, who she discovers, via a television monitor, is in bed with Frank. She then discovers Rocky, cowering in his birth tank, hiding from Riff Raff, who has been tormenting him. While tending to his wounds, Janet becomes intimate with Rocky, as Magenta and Columbia watch from their bedroom monitor.
After discovering that his creation is missing, Frank returns to the lab with Brad and Riff Raff, where Frank learns that an intruder has entered the building. Brad and Janet's old high school science teacher, Dr. Everett Scott, has come looking for his nephew, Eddie. Frank suspects that Dr. Scott investigates UFOs for the government. Upon learning of Brad and Janet's connection to Dr. Scott, Frank suspects them of working for him. Frank, Dr. Scott, Brad, and Riff Raff then discover Janet and Rocky together under the sheets in Rocky's birth tank, upsetting Frank and Brad. Magenta interrupts the reunion by sounding a massive gong and stating that dinner is prepared.
Rocky and the guests share an uncomfortable dinner, which they soon realize has been prepared from Eddie's mutilated remains. Janet runs screaming into Rocky's arms and is slapped and chased through the halls of the castle by a jealous Frank. Janet, Brad, Dr. Scott, Rocky, and Columbia all meet in Frank's lab, where Frank captures them with the Medusa Transducer, transforming them into nude statues. After dressing them in cabaret costume, Frank "unfreezes" them, from which they spontaneously perform a live cabaret floor show with Frank as the leader.
Riff Raff and Magenta interrupt the performance, revealing themselves and Frank to be aliens from the planet Transsexual in the galaxy of Transylvania. They stage a coup and announce a plan to return to their homeworld. In the process, they kill Columbia, Rocky, and Frank, who has "failed his mission". They release Brad, Janet, and Dr. Scott, then depart by lifting off in the castle itself. The survivors are then left crawling in the dirt, and the narrator concludes that the human race is equivalent to insects crawling on the planet's surface.

Question: What sort of event was happening at the castle?
Answer: Annual Transylvanian Convention
Alternative answer: a convention