Irish schoolteacher Sharon meets American advertising executive Rob in a London bar. They have a sweaty fling for a week and then Rob returns to the U.S. Soon after, Sharon discovers she's pregnant. She calls Rob, who returns to London to help 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. Rob endeavours to cook up a proper proposal to help Sharon feel better about the clumsiness of their situation.

Pregnant Sharon's hormones plague her with waves of horniness and sadness and a host of other fears. Sharon has a chance meeting with an old flame and Rob begins a clandestine friendship affair with Chris, the husband of a woman Sharon can't stand.

Still getting to know each other, Rob and Sharon continue to enjoy a vigorous sex life. Sharon's parents visit form Ireland and have a strained introduction to Rob. More health complications arise, jeopardizing Sharon's pregnancy and creating stress for both her and Rob.

Sharon and Rob plan their wedding while Rob plans a surprise birthday party for Sharon. Rob is fired via Skype by his Boston bosses and begins searching for employment in London. Sharon calls Rob's mom to invite her to their wedding and Rob's mom reveals that Rob has withheld a massive fact from his past from Sharon.

A very pregnant Sharon picks Rob up at the airport and tells him she's invited an old friend from Ireland, to be her maid of honor at their impending wedding. Rob decides to ask Sharon's brother Fergal to be his best man. Rob and Sharon split up to enjoy their stag and hen nights, both of which spiral out of control.