NOTE: This spoiler sent in by AarChon who says..."Pretty good movie, but the minor characters definitely steal the show."
The main character of the movie is a bear named Boog (voiced by Martin Lawrence), who we discover lives with Park Ranger Beth (Debra Messing) in her garage, with a nice sleeping pad, a stuffed teddy bear, and a private bathroom. When he and the ranger drive into town, they stop at the police station, where a sign is posted that OPEN SEASON starts in three days.
A hunter named Shaw (Gary Sinise) drives up to the station and parks next to the Ranger's jeep, which has Boog sitting in the back. On the hood of his truck is a buck, tied down and apparently dead. He tells the police chief that he ran into him on the road- a flashback shows the truck driving off the road to hit the young buck.
The buck, Elliot (Ashton Kutcher), wakes up, and works hard to convince Boog to untie him. Boog resists, saying that this is his town, and wonder why he should do anything. He also points out to Elliot that he has lost half of his antlers. Elliot responds to this by saying "Half deer, half buck, I'M A DUCK!" Boog finally gives in and cuts Elliot loose, just before the ranger drives off with him. Shaw comes out and sees Elliot get away, and blames Boog.
Boog gets home, and goes through his good-night routine, which involves fish crackers and a song about teddy bears. He's awakened in the middle of the night by rabbits hitting his window (the recurring rabbit theme is hysterical) and looks out to find Elliot with an armful of rabbits trying to get his attention. Elliot tells him that he's returning the favor, and setting Boog free- Boog responds that he IS free, though the image of a giant bear in a tiny, cluttered garage isn't convincing. Elliot resorts to luring Boog out of the garage with a candy bar, the sugar apparently intoxicating Boog. They then go to town, where they break into a convenience store and ransack it. This is a fun scene, with lots of different things going on, various foods being sampled, and the entire store being leveled. The police show up, Elliot bolts, leaving Boog behind, drunk on sugar. Boog goes home in the sheriff's truck, to face Beth. The sheriff suggests its time Boog go back to the wild; Beth reminds him that she could never teach Boog to fish. Shaw drives past the looted convenience store and gets squinty eyed with obviously evil thoughts.
The next day, Elliot is still in town, dumpster diving, when he winds up walking across the road in front of Shaw's truck. Shaw stops to avoid hitting him, realizes who and what it is in front of him, and then tries to run him over (again). Elliot runs away to a building where Boog is helping Beth put on a performance, Boog is behind a curtain on stage when Elliot comes through the backstage door and slams it in Shaw's face. Elliot begs for help, and wants to be Boog's buddy, Boog wants nothing to do with him. When Beth announces the mighty Grizzly, Elliot winds up stepping out, to the crowd's amusement, but then Elliot is pulled backstage and the shadow play of Elliot and Boog talking looks to the crowd like Boog is ripping Elliot apart. This is a clever scene; Elliot is tangled in rope, as Boog pulls it off him it looks through the curtain like he's removing intestines; Boog accidentally splashes a bucket of liquid, which looks like blood to the crowd.
The curtain is pulled back, Boog at this point is holding Elliot by the neck, and Shaw has seen the crowd running away and gotten in position to shoot. The sheriff grabs his gun as he shoots, but Boog goes down- when he does, we see Beth with a tranq gun on the other side of him. She also tranqs Elliot in the hind quarters, repeatedly.
Beth flies Boog and Elliot out into the wilderness, "Over the falls", so they won't be hurt once open season starts. She leaves Boog in a clearing, with his teddy bear.
Boog wakes up, and realizes he's not in town anymore. He takes this out on Elliot, by planting him in the ground by the antler, and starts hiking back to town. He goes in circles, repeatedly, always coming back to where Elliot is stuck in the ground; Elliot is shown going through various poses, always with his head stuck to the ground, trying to convince Boog to help him so he can go with Boog. Boog decides to climb a tree to look around; here we meet McSquizzy (Billy Connolly), who is a defiant, Scottish-sounding squirrel with an attitude; he won't let Boog up the tree, Boog asks the classic "you and what army" question, an army of squirrels appear on the branches of the tree, armed with nuts. Boog relents, and goes to another tree, where McSquizzy once again appears; when Boog tries to climb, he's pelted with nuts, and runs away with McSquizzy making fun of him.
Boog finally pulls Elliot loose, and Elliot starts leading him through the woods. Elliot meets a doe he recognizes; while he's talking to her, the rest of the herd shows up, and we find out that Elliot was thrown out of the herd just before he was run over by Shaw. Boog helps Elliot out here, facing off with Ian (Patrick Warburton), the alpha male of the herd, but he's mocked by the herd and ends up backing down. Elliot and Boog head out again through the woods, and eventually up a cliff.
As they are climbing the cliff, with Elliot talking nonstop, Boog looses his grip and falls, his fall finally broken by a bunch of tree branches. From the lowest branch, he looks down, sees nothing, and drops, just in time to land on a porcupine. Elliot shows up and helps Boog remove the porcupine; he calls Boog "Buddy" but the porcupine seems to think Elliot meant him; he follows Elliot for the rest of the movie, always saying "Buddy".
Boog and Elliot come across a huge beaver dam; Elliot shouts to the beavers that he and Boog are partners, and are going to have a dance routine in town; the beavers make fun of both of them. Boog is still not happy with Elliot, and doesn't want to be his partner; he tries to get some ducks to give him direction to town so he can ditch Elliot, but the ducks just have WWII style flashbacks to their flock being shot down by hunters.
Boog tries to "poop in the woods", an obvious reference to the old question about bears. He can't manage it without a bathroom, when he tries his privacy is invaded first by a number of rabbits, then by Elliot, then by two skunks, who he manages to irritate into gassing him.
At some point we see Shaw again, driving into the wilderness to hunt down Boog. He's convinced Boog is part of a plot by animals to take over the town, and this is reinforced when he drives by an RV which appears to be driven by a dog (actually sitting on the dash, I think a subtle reference to the fact that he's a Dachshund). A bit later, he sees the RV owners with their dog, trying to teach it a trick; he barges in and "rescues" them, explaining his theory about how the animals are taking over; they explain they don't need a rescue, they're here to find bigfoot. He wanders off, calling them crazy.
Back with Boog and Elliot, when they're ready to sleep, Boog needs Elliot to sing the Teddy Bear song to help him sleep. Elliot gets the tune right, but the words wrong; to me this was one of the funniest parts of the movie, as Elliot sings about a flatulent dwarf who is constantly needing to pee. Boog discovers at this point that his teddy bear is stuffed with fish crackers; he takes this as evidence that Beth really does care about him.
When they wake up, Boog has to remove a family of rabbits from his fur as he gets up. They set out though the woods again, but when they come across the same beaver dam, Boog gets angry as figures out that Elliot doesn't know the way home. Elliot explains that he thought that if Boog spent time with him, he'd learn to like him. Boog sets out across the dam, which leads to what is probably the best action sequence in the movie: the dam breaks.
The flood (with Elliot and Boog being carried along) washes through the valley, picking up the various animals that Boog has met so far. They also pickup Shaw in his truck; Shaw makes a remark about hunting and fishing at the same time, and starts taking shots at them on their log. Boog loses his teddy bear in the flood, and eventually everyone goes over a waterfall. They got away from Shaw, but they're now surrounded by angry animals who blame them for washing everyone "below the falls" with Open Season about to start. Boog storms off, leaving everyone to fend for themselves.
Boog finds a cabin, with the all-important bathroom, which he uses. He discovers that it is a hunters cabin, though, with trophies all over the walls- Shaw's cabin, in fact, and Boog hides under a table as Shaw comes in, with the broken piece of Elliot's antler, and Boog's teddy bear. Boog forgot to flush; Shaw realizes that he's not alone, and starts hunting Boog, who narrowly escapes, and runs off through the woods again, this time finding the road to town. As he's standing by the road, he's passed by a convoy of hunters, and has an obvious moment of indecision, whether to go to town, or to help the other animals.
He finds Elliot, and tells him he found the way to town, and safety; at the mention of safety, all the other animals insist on going along. With a flash to a picture of all of the animals stuffed in the garage, Boog decides that they'll stay and fight. Here McSquizzy reappears, insisting on joining in. This leads to another good scene in the movie, the animals fighting the hunters. The animals go to the RV owned by the Bigfoot watchers, and strip it bare. The wiener dog goes native and joins the forest animals. The actual fight is modeled after various war movies, including apocalypse now and Braveheart.
A squadron of skunk-carrying ducks gas the hunters, and squirrels mounted on deer pelt them with flaming (marshmellow) arrows fired from underwear strung across antlers. The hunters are driven away, and a canister of propane is dropped on their trucks for good measure, causing a chain reaction which destroys all their vehicles. Only Shaw is left, who goes hand to hand with Boog, who loses and retreats until Shaw appears to have shot and killed Elliot, at which point Boog goes berserk and ties Shaw up by bending his rifle around him. Elliot wakes up, unhurt, and the forest animals beat on the defenseless Shaw.
Ranger Beth arrives in her helicopter, and tells Boog he can come home now. Boog starts to go with her, then decides to stay. The movies ends with Boog sitting down next to Elliot and saying that they should stick together, at which point Elliot takes off after a fawn.