Carrie is gearing up for the biggest night of her life - her book-launch party. Naturally, Samantha serves as Carrie's publicist and party-planner. Miranda, who's back in the office and loving it, gets a timely call from her pre-baby ex-lover, Walker Lewis. Excited by the prospect of a replay of their one-night stand, she invites him to be her 'plus one' for the party.
While discussing the party particulars with her punchy publisher, Carrie meets the charismatic (and cute) writer, Jack Berger. After a flirtatious fast-food meal on a park bench, Carrie dares to invite Berger to be her 'plus one' for her party, but her hopes are shattered when Berger casually mentions he has a girlfriend.
Carrie tells the girls that she 'sparked' with Berger and laments the fact that he's unavailable. Miranda admits that she purposely neglected to tell Walker she has a baby. After discussing the whole 'plus one' problem, Carrie starts thinking about what she has (a job and an apartment) and what she doesn't have (a boyfriend). She wonders why we let the one thing we don't have effect how we feel about everything. She asks: Why does one minus a 'plus one' feel like it adds up to zero?
Samantha decides to splurge on a chemical peel to prep for the book party and emerges looking like an overripe tomato. Charlotte prepares for the party by bedding country club-ready Justin Anderson the third, her 'perfect plus one.' The scene is perfect until Bunny barges in spewing accusations and Charlotte is left minus one.
Carrie soaks in the glamorous party in her honor and thinks 'Who needs a date? - until she spots Berger sans girlfriend in the crowd. It dawns on her that she's lonely. Miranda, on the other hand, has lured Walker back to her home. However, when Brady's cries interrupt them mid-climax, Walker walks out. Miranda takes Brady in her arms and realizes that she's not the same person - she's plus one. On the way home, Carrie's driver reminds her she's got something to celebrate. Spirits lifted, Carrie realizes she likes knowing that there are men out there who can still give her that 'spark.'