When Irish school teacher Sharon meets American advertising executive Rob in a London bar, it doesn't take long for them to end up naked. After a sweaty, week-long fling, Rob returns to the States and his old life. A surprise call from Sharon with some "news" prompts him to fly back to London to help figure things out.

Sharon faces the reality of having a giant new roommate in the form of Rob, the man whose child she happens to be carrying. As Rob tries to fit himself into his new London life, he connects with an old school pal, Dave, who's evolved into a delightful scumbag, and meets Sharon's feral brother Fergal - all while trying to cook up a proposal to help Sharon feel better about their situation.

Pregnant Sharon's hormones plague her with waves of horniness and sadness, not to mention a bucket of new fears. Rob begins a friendship affair with Chris, husband of Fran, the insufferable friend Sharon loves to hate. Meanwhile Sharon is having a chance meeting with an old flame, which leads to a clandestine dinner.

Still getting to know each other, Rob and Sharon keep up a vigorous sex life. When Sharon's parents visit from Ireland and meet Rob for the first time, they are not prepared for Sharon's double-dose of news. Meanwhile, Sharon's doctor shares some news she's not prepared for, and Rob's secret friendship with Chris explodes in a way he never could have imagined.

As Sharon and Rob try to plan a wedding together, Rob suddenly faces an uncertain financial future. When Sharon calls Rob's mom to invite her to their wedding, her future mother-in-law reveals that Rob has withheld a secret from his past.

Rob and Sharon split up to celebrate their bachelor and bachelorette nights, both of which spiral out of control. When the big day arrives, Sharon's maid-of-honour is missing. With the pressure mounting and her stomach nearly bursting, Rob and Sharon must figure out a way to get through the big day - not to mention their wedding night - despite a major eruption.