The movie starts with a giant meteor hurtling through the depths of space. From there, we go to the quaint midwest town of Wheelsy, where new Sherrif Bill Pardy (Nathan Fillion -- Serenity fans represent, yo!) and his deputy Wally (Don Thompson) out at night measuring the speed of a passing...whipporwhill. While Wally reflects on his fading speed judgin ability, the meteor crash lands behind them...at the size of a basketball. The meteor cracks open, and inside we see a big pulsing growth.
The next day, things are getting ready for family fun weekend in town as the foul-mouthed mayor, Jack McReady (Gregg Henry) drives into work after cursing a jeep driver out in front of a little girl and her mother. Elsewhere, we see science teacher and former beauty pageant winner Starla Grant (Elizabeth Banks) trying to teach her students -- including Connie Strutemyer (Tania Saulnier) (the IMDB credits say Kylie, but I swear I heard them calling her Connie the whole film), who looks like she's out for Starla's former crown and is currently passing notes with one of the cuter boys in class. School lets out, and while Bill talks with crossing guard Margaret (Jennifer Copping) and her young son, Starla's husband Grant Grant (Michael Rooker) comes by to pick her up, although he gets jealous after seeing her talking with one of the male teachers. The pair leave and head for their house where that night Grant tries to work up Starla, but she's "not in the mood." Disappointed, Grant throws on his pants and walks out.
Sometime later, we see Grant drinking alone in the local bar when he is approached by Brenda Guiterrez (Brenda James). The two talk and she tells Grant how she's jealous that her fat sister could find someone to stick around and love her while she gets nothing but a baby to take care of. Before you know it, the two are out in the woods and about to have sex when some part of Grant (not that part, obviously) stops him and reminds him of Starla. Brenda starts to insult Grant but stops when Grant starts following a trail he just found. They eventually find the slug thing and Grant takes a stick and starts poking at it. A needle comes out of the top and fires at Grant, embedding itself in his chest before burrowing all the way in. Grant falls over, apparently dead, and Brenda starts trying to wake him up. A few minutes later, after the needle has dug into his brain, he does wake up and walks off with a glazed look in his eyes, leaving Brenda there wondering what's wrong with him.
The next morning, we see Starla waking up in bed, and she notices that Grant isn't there, although his truck is parked rather awkwardly in the driveway. For his part, Grant is downstairs spreading leaves and other forest stuff in a kind of bed on the cellar floor. He starts smelling meat and heads upstairs where he finds Starla waiting loooking rather beautiful, and she apologizes for last night and the two have make-up sex, although since Grant's not Grant, it's more like plain ol' sex. Later, Starla is leaving with a big smile on her face while unbeknownst to her, Grant is going through the fridge for any meat related product when he hears a dog barking outside. He goes to check it out and...
...we see Starla bragging to one of her co-teachers about the incredible makeup sex she and Grant had. That night, though, things get weird for Starla when she notices that Grant has put a lock on the basement door and later still when she's in the shower, with Grant outside ready to infect her, but he chickens out and runs off, saying he'll meet her at the hunting dance in town after he takes care of something at work. Confused, Starla heads over to the dance where Wally is giving Bill a hard time about his crush on Starla, to which Bill says, "Fuck you, fatty." Regardless, he still goes to talk to an impatiently-waiting-for-Grant Starla and the two start having fun. Meanwhile, Grant has gone over to Brenda's house and seems to be having sex with her when two tentacles pop out of his chest and he infects her with...whatever. Later that night, Starla arrives home and finds Grant there, only his face looks like roadkill. He waves it off though, saying it's a bee sting and he went to the doctor earlier -- which is where he was -- and he said it should clear up in a day or two.
The next day, Starla is riding home from work when at a stoplight, she sees a Lost Pets sign advertising the sheepdog from before. Meanwhile, Grant visits a barn deep in the woods where we see a chained Brenda, whining about being so hungry. Grant deposits a couple of bags filled with meat in front of her and she begins eating them ravenously as an even-worse looking Grant smiles. That night, at the Grant household, Starla is approached by Wally and Bill, who tell her that Brenda is missing and Grant was the last person to be seen with her. Bill leaves his card with his direct number on it and asks her to have Grant call him so they can sort this out. She says okay and goes inside, looking like she's about to cry. She looks at the basement door with the padlock on it and for whatever reason, breaks it open with a baseball bat. She goes downstairs and finds a mess of dead animals with their guts torn out, including the sheepdog she saw on the poster earlier. She runs upstairs and tries to call Bill, but is interrupted by Grant, who breaks in and starts trying to kill her with an extra long tentacle left arm. Just then, Bill and Wally come back with the rest of the police force -- seems Starla's call did get through. They gaze in horror and wonder at Grant, and he throws Starla at them and takes off.
3 DAYS LATER...
We find Bill leading his cops in a review of "Squidboy's" (that's Grant) latest appearances when Jack comes in asking if they've caught him yet. When he gets a negative, he asks how hard it can be to catch one guy with Lyme Disease. The others look at him incredulously, and he explains "You touch rat droppiings without washing your hands, then you eat something, and boom -- Lyme Disease!" The others ignore this and plan where to be that day for Grant's next likely appearance and with a couple of hunters -- a young white guy and a middle aged black guy -- they run out. As they're leaving, Starla comes up asking to go with them, saying they'll never find the still-missing Brenda unless Grant gets taken alive, and after a while, Bill's crush forces him to agree. First, though, they stop at the Strutemyer house to try get the literal man of the house Otis (William McDonald) to come with them, but he turns them down and goes back inside to his family of Connie, wife Ethel (Iris Quinn), and twin eight year olds Emily (Matreya Fedor) and Jenna (Amber Lee Bartlett).
Meanwhile, over at the farm of the day, everyone -- including Jack, for some reason -- is setting up to wait for Grant's appearance. That night, as Starla and Bill are recollecting for Wally a time when she was twelve and came over to his house and asked him to come to Hollywood with her to be the bodyguard to her famous actress, and he called the cops on her instead, Grant shows up, now looking more like "half-tree/half-worm-boy" He starts eating a nearby cow and the party comes out, with Starla begging him to come quietly, saying she will stay behind him, since "Marriage is a sacred bond for better or for worse." ("Much fucking worse," mutters Bill) Grant turns and starts to get out of there, but is cut off by the young white hunter -- who promptly gets sliced in half as all his guts spill out (TOTALLY the grossest scene I've ever seen in a movie). Grant takes off and the others follow him into the forest, eventually happening upon the farmhouse where Brenda is. They hear her in there, go inside...and find a 5,000 pound Brenda that looks like a human hot-air balloon. She asks them for some food -- a little possum, maybe -- and Bill says they'll get her to a hospital. Just then, she starts shaking and screams "They're tearing me apart!" just before bursting open as thoousands of little slugs come streaming out of her. Bill yells to cover their mouths, but it's no good -- only him, Starla, Jack, and Margaret survive, the slugs having taken Wally, Trevor, and the black hunter.
Meanwhile, we see Connie taking a nice relaxing bath while her mother tucks Jenna and Emily in and her father watches TV. Outside the bathroom window, we see a slug sneak in and make it's way over to her. It hops in the water, but just before it goes in her mouth, she wakes up and sees it. She screams and it starts going in, but she grabs it and pulls it out, getting a whole boatload of race memory in so doing. She throws the slug against the wall and cuts it in half with a curling iron, but then the house gets inavaded by dozens of more slugs, which manage to infect the rest of her family. She gets outside, jumps to the ground, and crawls into her dad's pickup truck, but since he has the keys and is inside the house, she's stuck...especially as the slugs manage to cover every surface of the truck.
Back at the barn, the survivors are gathering the bodies and Bill says that he's going to the station to get help -- like from the CDC or something. He leaves and as the others are tending the bodies, they start getting up and asking Starla why she ran. It soon becomes clear that they're speaking for Grant, and she tells them all to get away, threatening to shoot them. They laugh until she blows Wally's head off with a shotgun, and then it's a merry ol' chase through the woods -- except for Margaret, who gets eaten.
Meanwhile, we see Connie in the truck when her family stumbles out of the house, demanding that she join them (even though they're zombies). Just then, Bill shows up and Connie runs for him, telling him what happened. The two get into Bill's police car and get out of there just in time to see Jack and Starla cross the street in front of four or five zombies. Bill distracts them and Starla manages to pick up some kind of pole and kills one of them with it by bashing its brains in, causing Jack to go, "Bitch is hardcore..." Starla and Jack get in the police car, and after a minute of silence, Bill goes, "So...how's everyone's evening so far? Good?" He calls radio operator Shelby at the station and after reminding her not to nod over the radio asks her to get the CDC on the line for him. She says okay and they sign off just before a shitload of slugs drop out of the ceiling onto her. Connie speaks up and tells them the history she learned from her experience with the slug -- that these things have been moving from planet to planet for millenia, and the leader's real head is shaped like a needle, and that Grant is the first sentient host he's had. Bill calls Shelby back, and he hears, "Hello there, killer. I've got a nice big surprise for you. It's waiting just around the corner." He looks up and the car gets totaled by a big 18-wheeler, knocking him and everybody else out.
Some time later, Bill comes to in time to see a bunch of zombies making off with Starla's still unconsciouus body. He grabs Connie and pulls her out of there, but manages to lose Jack. He and Connie run for the police station, and he asks her if they kill Grant, will the others die, and she says yes. He tells her to hide in a nearby alley, and he runs into the police station to grab a grenade that just happens to be there. Inside, something moves in the dark, but he ignores it and tries to get an outside line. Naturally, the phones and radio are dead. Bill goes for the grenade but is suddenly attacked by a mutant alien deer that got infected back when Brenda blew up. He gets saved by Connie, who kills the thing with a fire extinguisher to the head. Bill grabs the grenade and thanks Connie, although he shamefacedly asks her to tell anyone who asks that he saved her, to which she replies, "Okay."
Back with Carla, she wakes up in her bedroom, alone and confused. She goes over to her bedstand, picks up a comb with a rather sharp handle, and pulls the bristles out and tucks it into her stocking -- instant dagger. We quickly cut to Jack and see him in a dark room. he takes out a lighter and gets it lit in time to see himself surrounded by zombies. They head for him and the light goes out. Sometime later, we see Jack sit up, apparently okay and being ignored by the zombies. He looks at a dead arm sticking out, goes over to it, looks one more time at the zombies, and then digs in. Upstairs, we see one of Grant's tentacles put on some Barry Manilow as Starla comes into the living room and sees a big blob Grant (totally gross) absorbing (or becoming one with) people. Outside, we see Connie and Bill watching this and Bill says, "Well, that is some fucked-up shit right there." Inside, Starla starts talking to Grant and calming him down, saying she will stick by him. Just as she's about to kiss the big blob, she pulls out the dagger and attacks him with it, getting thrown against the wall for her trouble. Bill takes this as his cue and runs in, followed by Connie, but Grant throws a couch at Connie and knocks her out against the wall and manages to hit the grenade into the pool out back, where it promptly blows up. Bill takes out his gun and starts shooting at Grant, but Grant gets one of his tentacles into Bill, knocking the gun to the floor. Outside, Bill is fighting off the other tentacle and spots a propane tank. He manages to grab it and shove the second tentacle into it. Grant pulls both tentacles in and Bill yells at Starla to shoot Grant. She picks up the gun, closes her eyes, and fires, at which point Grant explodes magnificentally.
A few hours later, the sun has risen on the town. We see Connie picking her way out from behind the couch as Starla wakes up covered in blood. The two girls stagger over to the inert body of Bill, who wakes up coughing as they're watching him. Connie tells him that he'll be okay, since he only got one tentacle inside him, and he says, "Oh...great." The three start walking out of town, and Bill speaks up, saying Connie should tell Starla how he saved her from that dear. Connie shrugs and says, "Oh, yeah...Bill saved me from a deer."