Jason (an advertising executive) and Julie (a charitable investment advisor), longtime best friends now in their mid-30s live in the same building in Manhattan. Not romantically involved, they are close friends with two childless married couples, the placid Alex and Leslie and the sex-obsessed Ben and Missy. During the next four years, after both couples have children, their marriages suffer. Following a chaotic birthday party for Jason at Alex and Leslie's place in Brooklyn, Jason and Julie discuss how it would be better to have children first, to 'get it out of the way' since 'time is running out', and only then meet the person that you wanted to marry. After more discussion, they decide to have a child together, despite never having had romantic feelings for each other, and then to continue to date other people, to find 'their one'. Although their friends predict disaster, Jason and Julie adjust to their new relationship with baby Joe far better than their friends had imagined.