A guest arrives at a bourgeois household and, in turn, seduces everyone: father, mother, daughter, son and maid. (Actually he doesn't seduce anyone but responds in a non-judgemental way to other people's desires - as Terence Stamp points out in the accompanying interview).
"Theorem" is one of the true classics of 1960s European art/auteur cinema. I imagine most people interested in this film already know it well. I'd just like to say that this is a fine new DVD edition from the BFI - good sharp print, nice booklet with review from 1968 & a new informative essay and the disc has an entertaining newly filmed interview with Mr Stamp, who worships Fellini & has a grudge against Pasolini almost as big as his grudge against Antonioni, but is perceptive about his character/role. And the fact is that Pasolini enabled Stamp to give his greatest performance.
As the interviews & essays discuss, the basic Marx-meets-Freud "theorem" that the bourgeois patriarchal family is upheld by sexual repression is pure 1968, but the film has proved timeless because of its unique mysterious & poetic quality. Also obvious, in retrospect, is that much of the film is really a representation of Pasolini's anxieties over his own homosexuality - mostly displaced onto poor Silvana Magnano, the housewife! Anyway, this is one 60s classic that actually improves with age - much imitated but never bettered - & well worth getting on this DVD edition.