In November 1995, Ángela, a university student in Madrid, is planning to write a thesis on audiovisual violence and the family.
While Ángela watches a violent film with Chema, Figueroa finds a tape hidden in the school's audiovisual archives.
The next day, Ángela finds Figueroa dead of an apparent asthma attack in the screening room and retrieves the tape.
At Chema's house, Ángela discovers the stolen tape is a snuff film in which a woman is tortured, killed, and disemboweled.
He is able to determine that the killer used a specific model of Sony camera with a digital zoom feature, and that the film was shot in someone's garage.
At the library, Ángela sees a handsome young man named Bosco using the type of camera Chema identified.
That night, Ángela has a dream that Bosco threatens her with a knife, performs oral sex on her while videotaping it, and then stabs her.
Castro, Ángela new thesis supervisor, questions her about Figueroa's death, showing her a security footage of her discovering the body and taking the videotape.
As Ángela is about to admit why she took the tape, Chema calls and tells her to leave Castro's office immediately, saying that he is involved in the snuff film.
In a room off the tunnel, they find shelves of video tapes like the one of Vanessa, indicating that many other women may have been murdered in other snuff films.
Castro tells that he only edited the snuff videos and did not murder anyone, but that he has to kill her and that her death will be painless and much quicker than Vanessa's.
Following the aesthetic of the American horror genre, Angela operates as the "Final Girl", or resourceful female protagonist that defies stereotypical feminine traits.
[4] Although Tesis fits the suspenseful mold of a Hollywood horror flick rather than its symbol-rich European counterpart, according to European film critic Marguerite la Caze, Tesis has a thesis: "human beings, no matter how well-meaning, are attracted to violence and death in all its forms".
[5] Joe Leydon assessed that Amenábar "provides an inventive plot and a sufficient supply of red herrings".