Suspiria (1977)

American ballet student Suzy Bannion arrives at the Tanz Akademie in Freiburg, West Germany, on a stormy night. As she arrives, a student, Pat Hingle, runs out of the school, shouting something Suzy cannot hear. That night, Pat is brutally murdered at a friend's apartment by an unseen killer.

Suzy meets the strict teacher Miss Tanner and the deputy headmistress, Madame Blanc. She suddenly falls ill and is put on a special diet. Strange events follow: maggots rain from the ceiling, and Suzy hears footsteps at night. Her new friend Sara, who was Pat's friend, tells Suzy she believes the teachers leave the building at night, but go somewhere inside it.

The academy's blind pianist, Daniel, is fired, and that night he is killed by his own guide dog in an empty square. Sara disappears, after being chased through the building and falling into a room full of razor wire.

Suzy meets a psychiatrist, Dr. Frank Mandel, and his colleague Professor Milius, who tell her that the school was founded by Helena Markos, a powerful witch rumoured to be its secret leader, and that a coven of witches draws power from her.

Back at the school, Suzy follows the footsteps and discovers a hidden corridor behind a mural with blue irises. She finds the coven, led by Madame Blanc, planning to kill her. In a secret room, she finds Helena Markos, who is invisible. She sees her shape in a flash of lightning and stabs her with a broken glass peacock feather. Markos dies, and with her the whole coven. The school bursts into flames, and Suzy escapes into the rainy night.

Who is Helena Markos? The ancient witch who founded the dance academy, also known as the Black Queen. She is the first of Argento's "Three Mothers", called the Mother of Sighs.

How does Suzy defeat the coven? She stabs Helena Markos with a glass peacock feather. Because the coven draws its power from Markos, killing her destroys them all.

What is the secret of the irises? Sara tells Suzy about "irises", and the key is turning the blue iris on a mural, which opens the door to the coven's hidden rooms.

What do the witches want? To protect their secret and their power. Anyone who learns too much, like Pat and Sara, is killed.

What are the Three Mothers? Three ancient witches in Argento's trilogy: Mater Suspiriorum in Suspiria, Mater Tenebrarum in Inferno (1980) and Mater Lachrymarum in Mother of Tears (2007).