Chris joins Nick, Emily and Violet in their living room to watch Nick's favorite movie, "What About Bob?" as Gary bakes cookies nearby in preparation for Marny's birthday. Sheila arrives to drop off Ernie; Nick encourages her to leave immediately, but Sheila senses that Ernie is about to speak his first word - and she doesn't want to miss it. She makes herself at home and then begins prompting the infant to utter "mama."
In a flashback to 2004, we see Chris and Nick in their college apartment. As Chris is serenading his then-girlfriend, Sage, with some awful rock rap, Emily bursts in to announce she and Nick are engaged. Chris is heartbroken that his roommate has broken their brotherly vow to not marry until they reach their 30s. But it turns out that Nick and Emily are pregnant.
Back in the present day, the birthday preparations are in full swing at Gary and Marny's apartment. Marny's thrilled at all the fuss - especially at the prospect of Gary getting his chipped tooth fixed before they take a family portrait. But Gary's resistant - he's grown to like his tooth the way it is.
In a flashback to 2005 we see the day when Nick and Emily met Marny and Gary. Both women are sharing a hospital room after giving birth to babies. Chris arrives to congratulate Nick and Emily on their little girl and to announce that he's broken up with his current girlfriend. There's a new girl on his radar, a tall looker named Sheila. Chris shows Nick a photo of Sheila on his phone. Nick declares her smoking hot - and demands that Chris pursue her.
We return to the present day, where Nick now has to admit that he is more than a little to blame for having Sheila in their lives. Emily wonders about Chris' old girlfriend, Sage; she searches for her in vain on Facebook.
We flash back next to a dinner party in 2006 just after Nick and Emily have moved into the same apartment building as their new friends Marny and Gary. Gary and Marny are hosting a dinner party to meet Chris and his now girlfriend Sheila. Gary, a hard-charging real estate broker, is sparring with their Russian nanny. After the nanny scolds Gary for not paying enough attention to his children, Gary fires her on the spot. Incensed, the nanny punches Gary in the jaw - chipping his tooth.
Back in the present, Sheila and Nick are still at each other. Nick adamantly tells Chris that while he may have set them up, he very quickly declared that they were not right for each other. Chris begs to differ. We flash back to Sheila and Chris' 2008 wedding. At the altar, Chris recites vows that clearly betray how much Sheila has tamed him. Nick pulls Chris aside to tell him he mustn't go through with the wedding; he declares Sheila evil. But Chris is in love and determined to move forward, even if it means losing his best friend from college. Nick refuses to give Chris the wedding rings. The two wrestle, with Chris finally extracting the rings from Nick's fist and storming out. Sheila bursts in. She's heard the whole dustup and declares that she and Nick are now lifelong enemies.
Next, in 2011 we watch a contrite Chris return to his old friends, this time pushing a baby in a carriage. After three years apart, Nick is still hurt about their lost friendship, but when Chris admits that Nick was right about Sheila, Nick turns warm and welcoming. Chris goes on to explain that he and Sheila are getting divorced; he's currently living in a hotel. Nick tells him that an apartment has just opened up in their building.
In the present day, Chris reminds Nick that having Sheila in his life is indeed his fault: Nick insisted on Chris moving into the building - and Sheila soon followed. Nick scrambles, blaming Ernie for being born and keeping Chris and Sheila together. Chris adds that really it's Violet's fault, since Emily's pregnancy ruined their guy pact to not get married. Sheila follows by blaming Emily for being "easy" and getting pregnant. Emily's heard enough and declares that she'll take the blame. But she's got a plan to fix it: she thinks she's found Chris' old girlfriend Sage. It turns out to be a false lead, but with Sheila's help, Nick finds an old slip of paper with Sage's phone number. Chris excitedly dials the number. As his old girlfriend picks up, baby Ernie utters his first word: Sage. Sheila's mortified.
Marny returns home from work for her birthday celebration. As Yoda and Clark head off to eat the cookies they've baked, Marny wants to know why Gary refuses to get his chipped tooth fixed. Finally the truth comes out. Gary reveals that the chipped tooth reminds him of the days when they were both working and he was a jerk. The nanny clocking him was a wakeup call and the chipped tooth a reminder. Marny gets it - and admits that she's much happier with who they are now. That chipped tooth doesn't look so bad after all.