LITTLE MANNOTE: This spoiler was sent in by Dapope.
The movie opens up with Calvin (Marlon Wayans) doing pull ups in prison. He gets to three and repeats the number 3 times before demanding another prisoner to count for him. Police come to his cell and tell him he is being released. As they bring him out, we see the inside of the prison. They open the door of the prison to reveal him to be a midget. We see his ride and wannabe rapper Percy (Tracy Morgan) on his way to pick him up. He picks up Calvin and tells him they need to get a diamond from a local jewelry store, which will pay each of them 50,000 dollars. Percy (P-Unit) says he needs it to start his record studio. He then puts in a recording of his. He begins to rap to the song being played, which is the beat of "Candy Shop" by 50 Cent. But his version, after being confronted by Calvin about it's similarity, is called "Butcher Shop".
Cut to a nice restaurant, where Vanessa (Kerry Washington) and Darryl (Shawn Wayans) are to meet. Darryl is talking to his friend (David Alan Grier), telling him to come out when the time is right. Soon Vanessa arrives and tells Darryl she has news. Right before she says it all, Darryl's friend pops out playing "Gonna Have My Baby". She says that she actually got a promotion, and he pops out singing "Celebration" before shouting out, "REMIX!" Eventually he is shooed by Vanessa, and it is revealed she doesn't want a baby like Darryl does. Cut back to Calvin.
Eventually they reach the jewelry store, where the diamond is located. They go inside the store, with Calvin in a duffel bag. Percy puts down the bag and begins to talk to a worker, who, after Percy tells her he is a rapper, begins acting "gangsta" to get his business. Meanwhile, Calvin goes under the table holding the diamond and removes the diamond by sawing a circle under the table and removing it. Calvin then gets back in the bag and crawls back to where Percy dropped him. Also, we see a dog get in the bag with Calvin not knowing. When Percy picks up the bag, an old lady notices her dog is missing and alerts security, leading to the diamond's disappearance being noticed. Percy runs out with the bag and they get in the car. As they take off, they only go in donuts, as a boot got put on their car. They then get out and run on foot as two police men chase them. They hide in a store while police find another black guy and begin beating him. Meanwhile, Calvin hides the diamond in a lady's purse to get rid of the evidence. Calvin and Percy meet up after police leave, believing they lost their man. They decide to follow the lady and her husband (Shawn Wayans) home.
When they reach their home, they discover the husband, Darryl (Shawn Wayans) wants a baby. This leads to the idea to hide Calvin as a baby and get the diamond back. Percy rings the bell and hides in the bushes. Darryl and his wife Vanessa (Kerry Washington) answer the door, finding the "baby" and a note from Percy asking them to take care for his baby because he could not. After an attempt to call child services, they discover they can't bring him in, as they're closed. They change his diaper, revealing a huge... well, you know. Vanessa's father lives with them, and adds a comedic edge. Also, he doesn't trust the baby. After Calvin coughs horribly, they decide to take him to the doctors.
At the doctors, they discover a scar from a knife fight or appendix removal (it's never revealed) and a tattoo, which they blame on bad parenting. After the checkup, Calvin steals the purse with the diamond in it as Darryl and Vanessa argue over turning Calvin into the police so they can find his parents. Darryl wants to keep him while Vanessa wants to get rid of him. Calvin, meanwhile, runs into a police officer outside the room, who treats him like a baby. Before he can check the purse, Vanessa finds him and grabs the purse. Right when she is about to turn Calvin over, he calls her "Mama", leading to her keeping him for the weekend. After bringing him home, they decide to invite their friends to meet the baby and loan them baby stuff for the weekend.
Cut to Percy going into a shady bar run by the gangsters who offered to pay them for the diamond. He threatens Percy, telling him he had 24 hours before his goons would get involved. Cut to Calvin.
When their friends arrive, they take turns trying to describe the ugly baby, such as "National Geographic Beauty". The women go into the kitchen with Calvin. Calvin pulls himself onto Vanessa's cute friend Brittany. She suggests breast feeding after he tugs at her breasts a few times. He says yes, expecting her to do it, but is sadly mistaken when Vanessa's less than cute friend Janet offers to do it. Calvin bites her and runs away. Soon we see their friends leave, with Brittany stopping to give Calvin a kiss, leading to him frenching her. All friends leave, and Vanessa and Darryl put Calvin to bed. Calvin decides to stay up and wait to take the diamond and leave, but is incapable of fighting off the lullaby, and falls asleep. He wakes up later that night and goes downstairs to call Percy and get the diamond. While on the phone, though, Vanessa's father, Pops, comes in, shotgun in hand. He starts threatening Calvin before Darryl comes in. He takes Calvin away, not believing Pops, and warms up some milk for him. He then sits Calvin down and begins an emotional speech about wanting to be a father. Vanessa watches as he talks, and finally laughs as Darryl and Calvin drink their milk. When asked why, she reveals the milk to be he ugly friend's breast milk. This causes both of them to spit it out. Cut to the bedroom, as Darryl and Vanessa begin getting romantic as Calvin watches. The next morning, Vanessa wakes up as Darryl comes in. She looks happy, saying she can't remember the last time they did it twice in one night. He looks puzzled as Calvin comes up from under the blanket. This leads the audience to come to the assumption that Calvin got it on with Vanessa.
Vanessa makes a nice breakfast for Pops and Darryl. Pops comes down while Calvin is watching a kids show. She gives Pops his breakfast while he turns the TV to the news. Right before they show a photo of Calvin, the suspect of the diamond heist, he cries out D-Rex, leading Vanessa to change it back. Pops begins to read the paper as Calvin eyes his breakfast. Soon he jumps over and grabs his food, one item at a time. Eventually Pops realizes his food is gone and blames Calvin. Vanessa brushes him off as he leaves for a diner. Darryl comes down stairs and grabs his breakfast as Vanessa leaves for work. He begins to feed Calvin who hides his face, prompting a game of peek-a-boo. Soon Calvin gets tired of it and smacks him over the head with a frying pan. As Calvin drags Darryl's body, he grabs the diamond out of her purse. Vanessa comes back in as she forgot her purse and wakes up Darryl from his unconsciousness. She yells at him for sleeping, and he looks puzzled. Calvin hides the diamond in a diaper bag while Vanessa leaves.
Darryl and Calvin go to the park and play multiple games, each one resulting with Darryl getting hit in the nuts. Eventually Darryl and his friends go for ice cream, leaving Calvin with the two other women, Brittany and Janet. Percy shows up as Calvin tries to find the right diaper bag. He grabs one as the moms find him, thinking he was trying to kidnap Calvin. They chase Percy down as he tries to run off with the bag. He gets beaten before getting away. Darryl arrives and hears the news, deciding to take Calvin home. As they load the car, Calvin begs for keys. He begins whining and crying until Darryl gives him the keys. Calvin gets in the car and takes off, leading Darryl to believe someone stole the car. He stops a mini van being driven by Molly Shannon and her kids and begs her to chase down the car. She does and drives like a mad man as she tracks down the car. After a lengthy chase, the car is stopped, and the police open the car to reveal Calvin. The police are convinced the car jacker got away.
We see Calvin tied up and Darryl debating going to a hockey game. They decide to go to the game with Calvin. At the game, Calvin takes the diamond and swallows it to hide it from Darryl. He then grabs a beer from the man next to him and drinks it to keep it down. The drink lady comes down and asks if he wants some more, and he gets some, as do Darryl and his two friends. Calvin says "Nice Ass" as the lady passes Darryl and she believes it was him, calling him a pervert. Calvin calls her a tramp, and she pours a beer on him. Calvin drinks the man next to him's beer again, leading him to believe Darryl did it. Soon a cheap check costs a visiting team's player a trip to the box as fans boo him. He then challenges all of them, and Calvin throws a beer at the man. The player, thinking it was Darryl, comes into the stands and Calvin attacks him. The man then throws Calvin to the ground and stomps on him. We see Calvin in bed as Darryl talks about not being a good father. Then he decides to throw Calvin a birthday.
Cut to Percy bringing the diaper bag to the gangster, who opens it to reveal nothing but dirty diapers and other baby items. He is then told by the mob boss that he'll give him more time, but he better deliver. After Percy leaves, the boss tells his men that he suspects betrayal by Percy's partner, and to follow Percy.
At the birthday party, Calvin picks a fight with D-Rex, played by Rob Schnider, by calling him a loser who has no money. Eventually Darryl gets Calvin away from the D-Rex to play football with the other boys and their dads. At the start, the other team, led by Darryl's jerk friend who stiff arms the children, are winning. Soon though, Calvin and another kid decide to kick it up a notch and take down the other team, leading them to win. We then see Darryl answering the door where Percy says he is Calvin's father here to get him. Darryl beats him up, which the mob goons see, leading them to believe Darryl was Percy's partner, and he took the diamond for himself.
Cut to Pops in a diner, who, while watching the news, sees Calvin as the prime suspect in the diamond heist. He decides to stop him.
Cut to Calvin at a table where he is given a cake and begins to cry, as he never had a birthday or parents. Pops shows up with a teddy bear, saying he was wrong about Calvin. Calvin goes inside where he sees Janet give her son Canton Oil, causing him to poop. Calvin decides to drink it and poop out the diamond. He drinks it and goes as Vanessa and Darryl watch. Darryl leaves Vanessa to change his diaper, where she finds the diamond, thinking it to be a diamond for her ring, which had none. (Explained earlier when Darryl said he was waiting till he could afford one for her) She decides to take him out to dinner, leaving Pops and Calvin alone. It is now that Calvin reveals to Pops that he is a criminal. He then beats up Pops and calls Percy, telling him to show up in an hour.
Cut to Vanessa eating with Darryl. Darryl tries to tell Vanessa that he didn't get that for her, but he stops when she says she wants to adopt Calvin.
Vanessa and Darryl come home to find Pops strangling Calvin. Vanessa then calls the retirement home, who come to take away Pops. He shouts "Check the Bear!" Before being taken away. Vanessa puts Calvin in bed as she cries and goes in the bathroom. Darryl remembers the bear comment and finds the bear, revealing it to be a camera. He hooks it up to the TV, and watches as Calvin reveals himself not to be a baby to Pops on screen. He runs upstairs to tell Vanessa, both of whom discover Calvin to be missing as is the diamond.
Cut to Calvin and Percy in the car. Percy says that the mob boss thinks Darryl betrayed Percy and he has the diamond, so they are free to keep the diamond. Calvin realizes Darryl and Vanessa are in danger and he needs to go back. He does and finds Darryl, where the two confront each other about him lying about being a baby. Calvin apologizes, and before it can go any farther, the mob boss and his two goons bust in. They threaten to hurt Calvin and Darryl. Throughout many events, Calvin knocks out the goons while the boss threatens Darryl. Calvin comes in and throws a plane that hits the boss in the nuts. The police come in (Darryl called them after Calvin was revealed to be a fake) and arrest the mob boss and his goons. Calvin returns the diamond to the police and they tell Darryl he will receive a 100,000 dollar reward. After the police leave, Darryl and Calvin have a tearful goodbye, where Calvin reveals he liked being their kid. Darryl follows Calvin outside and they decide to go to a bar.
During the credits, though, we see Pops and Calvin looking at Vanessa and Darryl's baby. Calvin says it looks just like his father, and it is revealed to be Marlon Wayans face on the baby.