The film is about Joel Campbell (James Spader), a retired FBI Agent in Chicago who is struggling about failing to capture a Serial Killer, David Allen Griffin (Keanu Reeves). He attends therapy sessions with his female psychologist (and love interest) Dr. Polly Beilman (Marisa Tomei). When he gets back to his apartment, Joel finds that a girl who lived in his building was murdered. He fails to find any significance or connection in the murder to himself until he finds out that a picture of the victim was sent three days ago before the murder. To his room.
The serial killer, David Allen Griffin, has followed Joel to Chicago, his mind set on playing a fun game of cat and mouse with him. Joel is back on the case to catch him, no matter what it takes.
- Antagonist Title: David Allen Griffin is the titular Watcher.
- Attention Whore: Griffin suggests this is his reason why he's so attached to Joel, who he sees as the only person who takes the time to acknowledge him.
- Ax-Crazy: David Allen Griffin when he is about to kill his victims becomes very quiet and focused on killing, getting great pleasure out of doing so.
- Bound and Gagged: David Allen Griffin's first victim and, much later in the film, Polly.
- Depraved Homosexual: David Allen Griffin, who kills only women and is heavily attracted to Joel.
- Even Evil Has Loved Ones: Subverted. David Allen Griffin feels like he has a crush on Joel, but whatever love he has for him feels more like a creepy obsession than legitimate affection.
- Evil Wears Black: With the exception of one scene where he has a brown coat on, the serial killer Griffin is almost always dressed in black.
- Facial Horror: At the end, after Joel jumps into the water, he gets a glimpse at the half-burnt face of Griffin's corpse.
- Faux Affably Evil: David Allen Griffin seems like a very nice, affable man; when he introduces himself to Joel in person, he even offers him a bottle of beer. However, Griffin is a psychopathic serial killer who doesn't mind killing people, specifically women, and his love for Joel is more akin to a creepy obsession.
- Friendly Enemy: Subverted. Griffin acts very friendly when he's around Campbell, but later on tries to murder him.
- Gunman with Three Names: While not a gunman, Griffin is referred to as David Allen Griffin throughout the movie. Given that he's a serial killer, it's justified.
- In Medias Res: The movie starts on the last ten minutes of the movie, then cuts back to the beginning to show how the characters got there.
- Kick the Dog: Griffin first introduces himself to Joel at his fiancée's grave, and gives this gem:Griffin: I mean, is this a satisfying relationship ? You know, she loves me. She's decomposing. She loves me. She's decomposing.
- Mr. Fanservice: What do you expect when it stars Keanu Reeves as a homosexual Serial Killer?
- Murder the Hypotenuse: What David Allen Griffin does to every girl Joel likes. He first murders Joel's ex-lover because Joel was going to retire to be with her. Then he tries to kill Joel's female psychologist, because she's coming between them. In fact, in the movie, he even comes out and says that he did it because they got in the way of their relationship.
- No Sense of Personal Space: David tends to hug and touch Joel, despite Joel hating it.
- Police Are Useless: So much so that they fail to miss the killer's target when they're facing her!
- Retired Badass: Joel Campbell, a former member of the FBI who retired and became a detective.
- Serial Killer: David Allen Griffin, who loves stalking and killing women in various cities.
- Stalker with a Crush: David Allen Griffin is a textbook case. C'mon, just look at the title.
- Villainous Breakdown: Subverted. Griffin almost has one when Joel tells him that he's working on other cases involving serial killers, and tries to strangle Polly. However, Joel distracts him by letting him know he appreciates him.
- Worthy Opponent: Griffin considers Joel this after trying so hard to catch him.