NOTE: This spoiler was sent in by Emily.

Kate Holbrook (Tina Fey) is a career woman for a Health Food company, who’s never focused on her personal life. The movie opens with her revealing way too much about her desire for children to a terrified first date, who quickly leaves. At 37-years-old, she’s finally decided its time for her to have children, only to be told by her doctor that her T-shaped uterus gives her a 1-in-a-million chance of conceiving.

After trying to conceive for the ninth time and unsuccessfully trying to adopt, her sister Caroline (Maura Tierney) suggests that she try getting a surrogate mother. After some thought, she visits the Chaffee Bicknell agency, an agency that specializes in finding surrogate mothers. Chaffee Bicknell (Sigourney Weaver) explains the procedure and that the cost is an outrageous $100,000. (There’s a running joke throughout the movie in which Kate makes blatant comments about Chaffee’s age and how gross it is that she’s just starting to have children, while Chaffee gloats of how young her uterus is.) Kate agrees to the terms.

Out in front of her apartment building, Kate anxiously awaits the arrival of her potential surrogate. Her doorman Oscar (Romany Malco) makes really funny comments about her having a “baby mama” just like him. A busted-up SUV pulls up to the building, and out steps Angie Ostrowski (Amy Poehler) and Carl Lewis (Dax Shepard).

In Kate’s apartment, Angie and Kate begin to get to know each other, while Carl makes himself comfortable—he stretches out on the couch and goes through her drawers. Angie, after reading Kate’s sunsety aura, decides that she wants to have Kate’s baby.

In a very funny, slow motion montage played with “Endless Love” in the background, Kate and Angie are shown going through the implantation process, which involves a very large syringe.

Afterwards, Kate drives Angie home because Carl was trying to win tickets over the radio and couldn’t leave the house. Angie and Carl’s home is dramatically different from Kate’s. It’s very messy; they leave rotting food in the refrigerator, clothes all around the house, and a large pet iguana loose. Kate leaves some health food in the fridge, and is startled when Carl opens the door, while still using the bathroom, to talk to Angie. She tells Angie to keep in touch and to take a pregnancy test in about two weeks.

Kate, in the meantime, is also busy with work trying to find a location in Philadelphia for the newest branch of her company’s chain. While scouting locations, she stops in a Jamba Juice-type shop, where she meets Rob (Greg Kinnear), the owner.

One night, she gets a knock at her door. It’s Angie, who now needs a place to live since she’s left Carl for cheating on her. Kate obliges for the sake of the baby, whose sonogram picture Angie gives her. Oscar comments that it looks like a squirrel (important later). Kate begins baby-proofing the apartment the next day. Angie tries to use the bathroom, but can’t get the toilet open, so she pees in the sink. Kate passes by, and is very upset by this.

While living together, they begin to argue over their different lifestyles. Kate is a very organized person who tries to control everything in her life. Angie is the exact opposite—she eats junk food, drinks soda and Red Bull, and leaves food and clothes all around the apartment. Kate forces Angie to start taking better care of herself by making her drink water, take vitamins and prenatal pills, and eat healthier foods. Their biggest arguments are about Angie leaving gum under Kate’s coffee table and Angie dyeing her roots. They have a heart-to-heart where Kate explains her envy that Angie can have children while she can’t. They also begin taking Child Birth Classes, with an instructor who can’t pronounce certain letters, so the word “delivery” sounds like “dewiverwy”. (Very funny scenes all of them)

At the groundbreaking of the new location, Kate holds a press conference to address the concerns of the Small Business Owner’s Association and local community. There, she again meets Rob, where sparks fly but nothing happens.

As Angie is walking home one day, she finds Carl waiting for her. Here, Carl tells her that she’s never going to get away with lying to Kate. She’s not really pregnant, it seems, and the whole thing was a scheme to get money from Kate. (Their first check was for $10,000.) The sonogram picture that looked like a squirrel really was a squirrel. To continue the charade, Angie buys an inflatable stomach and fakes morning sickness.

To celebrate how well things are going, Kate and Angie decide to go clubbing. Angie dresses Kate in a sexy black dress, and gets her to drink a lot while in the club. Angie tries whenever possible to sneak in drink as Kate starts to loosen up. Still a little buzzed from the alcohol; Kate stops by Rob’s shop, where he asks her on a date. As Angie arrives home, Oscar spots her messing with her “stomach”. He confronts her about it the next day, and encourages her to come clean with Kate.

Before Angie can come clean, Kate leaves for her date with Rob. He takes her to a vegan restaurant, where they are served yeast and algae. Realizing that neither one of them was a vegan, they leave and end up eating Philly cheese steaks instead. Rob isn’t even bothered by Kate’s very specific order. They talk and when Rob mentions that he was a corporate lawyer before (important later) and has a daughter, Kate doesn’t say anything about Angie. Instead, she kisses him and spends the night at his apartment.

The next day, she takes Angie to her sonogram appointment, where Angie learns that she is, in fact, pregnant. While Kate’s on another date with Rob, Angie tells Oscar about her dilemma and reveals that the baby isn’t Kate’s, it’s hers and Carl’s, and though she knows Kate would be an excellent mother, she wants to keep the baby. She even tries to sneak out, but is stopped by Oscar, who tells her that he won’t let her do that to Kate.

Kate continues to see Rob, while Angie begins to improve herself, eating better and doing yoga. Kate still hasn’t told Rob about her situation with Angie, who he thinks is her sister. He later mentions that he thinks only dumb, rich people pay thousands of dollars to places like Chaffee Bicknell after he reads an article about the agency in the paper, which causes Kate to hold off mentioning the baby even more.

At the baby shower for Angie and Kate, Carl arrives unexpectedly exposes Angie’s lie. She then reveals to everyone that she’s really pregnant, but that the baby may not be Kate’s. Carl is shocked to learn that he may be a father, and Rob, who also arrived unexpectedly, learns the truth about Angie.

Kate goes to Chaffee Bicknell and finds out that there’s nothing she can do until she finds out who the baby’s mother really is. If it’s not her, then the agency will refund her all of her money. She drops Angie off at a hotel, where they argue and insult each other. Kate goes back to her old self, focused on work, especially the new store that has opened by now.

In court a few weeks later, Rob arrives to defend Angie while Kate represents herself. Carl shows up, and is told to remain quiet by the judge. Angie makes a moving statement to Kate about how knowing Kate has improved her life, and asks Kate’s forgiveness. Rob also makes a moving plea to Kate and the court, which the judge calls pathetic. The judge reveals that the baby is… Angie and Carl’s. Kate runs from the courtroom crying. She and Rob later reconcile outside the courthouse, but she leaves to go talk to Angie.

While they are talking, Angie’s water breaks. Kate drives her to the hospital. Angie creates the biggest scene going from the hospital entrance to her delivery room, grabbing strangers and screaming obnoxiously in pain. Along the way, Kate sees Chaffee Bicknell there, having birthed a set of twins. She scoffs in disbelief and joins Angie in the delivery room.

As Angie’s delivering the baby, Kate faints at the sight of the crowning. (Note: crowning is not shown) She wakes up in a hospital bed, with Caroline and Rob next to her. Caroline informs her that Angie had a girl and called them to be with Kate.  Kate’s doctor comes in and asks them to leave before informing Kate that she’s eight weeks pregnant. Kate is shocked but excited, and immediately tells Rob, who faints at the news. Kate visits Angie and her daughter, Stefani (yes, after Gwen). Angie tells Kate that she’s known she’s pregnant; she read her aura, and is very happy for her.

One year later, they celebrate Stefani’s first birthday. Kate and Angie are good friends. Rob and Kate are together and have a daughter. Carl is also there, having just begun parenting classes and gives Stefani a pocket bike as a present, which Angie is excited by.