The story goes something like this:
An out-of-work puppeteer (John Cusack) takes a job at a corporation. One day he discovers a small door hidden behind a file cabinet in his office. He crawls inside and finds that it is really a portal that allows him to become actor John Malkovich (played by John Malkovich!) for 15-minute intervals, after which he wakes up in a ditch on the side of a highway. Cusack tells a co-worker (Catherine Keener), and together they hatch a money-making scheme wherein poor slobs can enter John Malkovich for a price -- a bizarre vacation, of sorts.
But problems arise as people start using the portal to sleep with other people as John Malkovich. Soon Catherine Keener is having an affair with Cusack's wife (Cameron Diaz, looking engagingly ugly), who is having an affair with John Malkovich, who is sleeping with Keener. Meanwhile, Cusack is going out of his mind with jealousy, and John Malkovich himself is wondering why he keeps losing control of himself.
Malkovich has always been a creepy actor (some would even call him evil), and over the years he's developed a sort of cult following. This, of course, makes him a great target for lampooning, and Being John Malkovich looks to do just that. Of all the actors to choose from, he is the perfect subject -- scary, intense, and the last one you'd expect to make fun of himself (early reports have him calling himself an "overrated balding moron" during one scene).
This is going to be a great movie.