The movie starts off with photos of Costco's past employee of the months. Every employee working there is doing their job except for Zack (Dane Cook) who is in the lounge drawing over Vince's picture who has won Employee of the Month 17 times in a row. Zack is a box boy who spends most of his time slacking off and riding in his skate shoes. He makes his way to his buddies Lon (Andy Dick), Russell and Iqbal who also slack off a lot and play poker all the time.
We are then introduced to Vince (Dax Shepard) and his sidekick Jorge (Efren Ramirez - Pedro from Napolean Dynamite), who notice the picture of him in the employee of the month frame has been drawn over. Vince knows Zack drew over it so he replaces the picture with a new one and looks for Zack. Once he comes across Zack, they both have a thing against each other and throw insults at one another. Vince tells Zack that he will always be a box boy while he'll win again for Employee of the Month and become manager some day.
The whole store is then called to a store meeting where Glen the manager and his assistant Dirk announce that whoever will get employee of the month next month that they will win a new Malibu car which is more than likely going to be Vince from the looks of it but he encourages others to try and win. Glen also announces that they will have a new employee working the next day named Amy (Jessica Simpson). At the end of the day, Zack goes to his house where he also lives with his grandma.
The next day, a very lazy Zack tries to get up for work in the morning. That day at work, Zack meets Amy and is very much attracted to her. Zack tries to show Amy around but Vince comes in and steals Amy away from Zack. Zack orders for Russell to get Amy's personal file to see what she's really about. Russell manages to get the file by bribing the lady that works with the files with broken butterfingers. Zack meets his three buddies up in their secret hang out place which is up on one of the aisles in between all the grocery supplies. Russell tells Zack that Amy has a thing for guys who win employee of the month and that she left her last job because of relationship problems involving guys who won employee of the month. Zack gets very excited and tells the guys he's going to win Employee of the Month. The guys don't believe him until Vince overhears everything and challenges him which means he can never be late, not get any complaints and has to always be ahead of everyone.
That very day, Zack sets all his alarm clocks to 7: 58 and the next day is up and ready for work on time. Vince is shocked to see Zack on time for work and is a bit worried when Zack also gets the managers attention. Things don't seem to be going well with Amy who is wowed by Vince and his scanning abilities in which he is the fastest cashier. Zack steps it up by getting Lon to purposely drop a glass of food in one of the aisles and for Zack to run and clean it up but is beaten by Vince. That day instead of Zack getting a star Vince gets it. 29 more days to win.
Vince has a date with Amy that very night but Zack won't let Vince get away with it so he steals his wallet without noticing leaving Vince with no money.
That night, Zack is hanging out with his buddies at Iqbal's house. Iqbal has 22 kids (supposably, we never know but he has quite a bunch). While Vince is on his date, he doesn't have any money to pay but the waitress lets him off free without paying since she has a thing for him from the looks of it. Vince takes Amy home and tries to let himself in her house but Amy won't let him in but Vince manages to get a forced kiss out of Amy in which Amy is turned off by and just goes into her house.
That next day, Vince and Jorge are inside Vince's 81 Honda listening to music while Vince is making a big deal out of his Honda. Vince then is at the lounge where Zack makes it to work on time again and gives back Vince his wallet. Zack once again tries to find a way to get a star for the day when Semi, the security guard tells him that there is a child missing, Zack jumps at the opportunity but the child is throwing balls at them with the ball throwing machine but Vince comes to the rescue by knocking him out with one of the balls and bringing him back to his mom. Vince gets the star for the day. As the days go by, Vince keeps getting the stars until Zack finally just gets really frustrated and loses hope. Iqbal gives him some encouragement. Zack is then helping out a lady with a coffin and offers to take it to her car since its heavy but the lady insists to take it herself. The lady then goes up to the assistant manager and tells him that Zack is a really good employee which earns Zack a star for that day. Vince gets hysterical about it while Amy seems impressed by Zack and wants to hang out with him tonight.
That night, Zack and Amy are hanging out at Cosco, the store is closed of course in which they play golf together and share a time of talking in which they both find out they have a lot in common. Amy is slowly falling for Zack but is worried he is too much of a slacker who still lives with his parents, Zack also finds out she has huge ears.
The next day at work Zack is in a very happy mood but not the manager who announces that his brother who is also the founder of Costco is coming to check up on the store. He also announces that one of the cashiers has died and someone has to take their place, Zack volunteers which gets Vince very mad since it takes several months to know your way around the cash register but Zack manages. Glen's older brother comes in who happens to be a midget and is very strict when it comes to the service at Costco. He walks past Zack but seems OK with him at the speed he's going in his register which earns Zack a star for the day. The manager gives Zack a card to go into the cashier's lounge but his friends don't seem to happy about it and miss him very much.
That night, Zack has another date with Amy so while Zack is on the date Vince and Jorge sneak into his house and sets the alarm clocks forward so Zack can be late for work the next day.
The next morning, Zack wakes up and realizes his alarm is wrong and he only has 10 minutes to get to work. Zack rushes to get to work while Vince is standing right beside the punch ins and is waiting for Zack to show up late. Iqbal notices Zack hasn't come in yet and is a little worried. Zack makes it just exactly on time but can't find his card which causes him to be late but then we find out Iqbal had already punched in for him before he can be late. Zack tries to get even by putting Vince's Honda for sale and an old man buys it from him but Vince has no choice, either the customer buys his car or the customer is going to write a complaint so Vince lets him go off with his car. Jorge gets upset at him for selling the Honda since he wanted to buy it from him and tells him he's no longer friends with him.
As the days go by, Zack keeps getting the stars until he reaches 14 to Vince's 15. Zack has one more chance to catch up so he volunteers for the softball game their going to have so he can earn that star but has to cover for Iqbal at the same time. He covers for Iqbal but then leaves while he's on the shift to make it to the softball game. Zack comes in just in time as Costco had been losing and Zack manages to pick up the score and win the game which Zack gets the star for the day so now the title for the employee of the month is now tied and it will be settled with a final showdown the next day between Vince and Zack to see who is the fastest cashier.
We later find out that Iqbal has been fired because Zack was supposed to cover his shift but left for the game. Zack loses all his friends and later on that night, he's eating with the manager, Vince and Amy at a restaurant while his friends are also there but in another table. Zack goes up to them to apologize but they refuse his apology since he only wanted to win employee of the month to screw Amy. Amy hears this and gets upset so she asks Zack if its true and he says yes. He finds out that Amy does have a thing for the employee of the month but the employee of the month at her last job was her ex boyfriend but he was a jerk who only wanted to get in her pants which is exactly what Zack wanted to do, so she leaves.
The next day, Zack quits his job since he lost everything but his grandmother encourages him to at least try and get things back to the way they were. Zack stops by Iqbal's house which Lon and Russell are also at and apologizes but Iqbal thanks him. It turns out that since he lost his job he will be receiving checks for his children. Zack gets the guys to come with him to work to challenge Vince so they come along so that Russell can again bribe the file girl with Butterfingers to get Zack's file before he can be officially fired. That day at work, Jorge shows up in his new yellow Honda and he makes up with Vince to help him with the final showdown.
Before Zack and Vince go at it, Zack apologizes to Amy and thanks her for making him change to a better person and then Zack and Vince go off, Vince manages to be in the lead but Zack catches up to him. They both have untagged tires so they race to the where the tires are to get one with price tag but along the way they are faced with other obstacles and they both manage to complete them but as they both race back to their registers Vince has already scanned the last product which makes Vince the winner. Zack isn't too proud and neither is Semi so he goes back to the security room to check to see if Vince had cheated. Vince is then declared employee of the month and has won the new Malibu but then Semi interrupts everything to show everyone that Vince did in fact cheat because he forgot to scan one item and tells everyone that he had been giving away free stuff. The managers aren't too happy with that so they fire Vince and they declare Zack employee of the month and he finally gets a kiss from Amy while Semi grabs Vince and drags him outside by his belt buckle.
Six weeks later...
Vince is inside Jorge's Honda and Jorge is now talking about how cool his car is. We find out that Vince is now working at this new store but has a thing attached to his foot that he can't go a mile away from the store in which he does and the device in his foot goes off.
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