You already know the premise: A jilted/disillusioned–with-love-20/30-something revisits his/her past lovers to understand why he/she can’t find love, and finally realizes that her true love has been right there all along, in one form or another.
In this case, the Disillusionee is Ally Darling (Anna Faris), a recently-fired-then-slept-with-her-boss 20/30-something who reads a magazine article that says women who’ve slept with over 20 partners tend to not find true love. Ally is at number 19. Fearful of the consequences, she makes the decision that the next new guy she sleeps with will be the guy she marries. She also decides to seek out her former boyfriends/casual hookups, hoping to reignite some of the past’s embers and hence keep her number at 19.
There’s more than a passing similarity to High Fidelity in the premise and also in the choice to have several notable comedians—Andy Samberg, Parks and Rec’s (and Faris’s real-life husband) Chris Pratt, the BBCOffice’s Martin Freeman, Zachary Quinto (not so much a comedian, but hey)—and the perfect guy who’s been there all along is her next-door neighbor Colin (Captain America’s Chris Evans).