Who become lovers?
Matthew is an American exchange student who has come to Paris to study French. While at the Cinémathèque Française protesting the firing of Henri Langlois, he meets the free-spirited twins Théo and Isabelle. The three bond over a shared love of film.
After dinner with their parents, Théo and Isabelle offer Matthew the chance to stay with them while their parents are on a trip. Matthew accepts, considering them his first French friends.
Matthew becomes suspicious of their relationship after seeing them sleeping nude together; he soon discovers that they accept nudity and sexuality liberally. After Théo loses at a trivia game, Isabelle sentences him to masturbate to a Marlene Dietrich poster in front of them. After Matthew loses at another game, he is seduced to take Isabelle's virginity. The two then become lovers.
Matthew begins to accept Théo and Isabelle's sexuality and his time living with them soon becomes idyllic. The three re-enact a famous scene from Bande à part by "breaking the world record for running through the Louvre", and Matthew and Théo engage in playful arguments about Charlie Chaplin and Buster Keaton, Eric Clapton and Jimi Hendrix, as well as the subject of Maoism, which Théo fervently believes in. 
During this time Matthew begins to pursue a relationship with Isabelle, separate from Théo. Matthew and Isabelle leave the house and go on a regular date, which she has not experienced before. Théo retaliates by inviting a companion up to his room, upsetting Isabelle. She distances herself from both Théo and Matthew, only to find them next to each other on Théo's bed when an argument between the two turns erotic.
Matthew Isabelle