This is a wonderful exploration of what happens when your imagination runs away with you.
Campbell Scott and Hope Davis are married dentists (Dave and Dana) who share a practice somewhere in New York State. Dave sees Dana in an illicit moment, but all he sees of the "Other Man" is the back of his head. But he suspects she's having an affair and from that moment on, every minute she's late, every word she says, is suspect. What does she mean by that? She's doing "paperwork"...what is she really doing? He imagines his wife having sex with everyone in the dentist's office...and then he imagines that he's wrong, his wife loves him. and that she wants to seduce him...those thoughts are almost worse than the ones where she scorns him.
To help him deal with the situation, he creates an embodied Id in the form of a particularly surly patient; Denis Leary inhabits this character, rather than merely portraying him. He is not pure evil, for such a thing could not exist on Earth...but he is very, very close to it. He says the things Dave cannot, and he says them with such cutting accuracy that, sometimes, Dave accidently blurts them out as well.
"Why don't you just ask her if she's having an affair?" he keeps asking Dave. "I can't. Because then she'll tell me and then I'll have to deal with it," is Dave's resolute response, and it never wavers no matter how many times he is asked.
This movie also contains one of the most accurate sick-with-the-flu scenes I think I've ever seen. Many actors can portray "drunk" effectively, but it takes a really talented person to convincingly play "sick with a fever and stomach flu and on the verge of vomiting," but Campbell Scott really pulls it off.
(There's quite a lot of very realisting vomiting in the middle of this movie...best to keep that in mind when planning a romantic dinner-and-a-movie date).
Towards the end, the line between reality and Dave's tortured imaginary world becomes more and more blurred. Until finally, he has to confront his wife, send Denis Leary packing, and get his life, or at least a shadow of it, back.
This is an amazing movie. It's a cerebral thing, a movie of the mind...but at the same time there are bits that just make you laugh uproariously. It's a tightrope-worthy balance, between the maudlin and the surreal, and it's managed brillliantly.