Zack (Seth Rogen) and Miri (Elizabeth Banks) have been friends forever. They've been roommates for years. But that's the extent of their relationship. They've never dated or had sex and the subject has never really come up. It's like they never heard Billy Crystal's theory in When Harry Met Sally.
Zack and Miri are not exactly upwardly mobile people, and they've been floating on their collective debts for some time. That's bad news for roommates. The water in their apartment is turned off. Next is the electricity, which the pair tries to circumvent by setting newspapers on fire in an oil drum they've put in their living room; it's kind of an indoor Hooverville. They fear that they'll be evicted between Christmas and New Year's if they can't make the rent, which they can't, and since they live in Pittsburgh and not Palm Springs, being out in the cold on Christmas means really being out in the cold.
The solution, it seems to Zack, is to make a porn movie. Having just attended their ten-year high school reunion, he figures that everyone there would want to see them have sex just because. His logic is a little flawed, because Zack believes everyone wants to see anyone have sex, even Rosie O'Donnell. And that's not true. Not everyone would pay to see that, and personally, I'd pay not to see it. After a little arm twisting, Miri agrees to make the adult film, and Zack goes out and recruits a cast and crew. The process, functioning here as a long montage, is more or less what you'd expect - auditions, gaffes, sex scenes, etc. - and the "make a porno" part of Zack and Miri Make a Porno is just not very interesting, especially if you've ever had some of the sex yourself or have ever "accidentally" watched pornography. At the very least, it's not as interesting as the relationship between Zack and Miri should be. That's should be.