Abigail (Christina Ricci) is a single mother of two who becomes pregnant after a one-night stand with Ben (Hamish Linklater). She's mature and down to earth; he suffers from Peter Pan syndrome and tends to put his foot in his mouth (trying to talk her into getting an abortion: “no one is going to want someone your age with three children”).
In spite of his reaction, Abigail has sex with Ben again the same night she breaks the news to him; perhaps she thought she wasn't going to get any more pregnant than she already was. The next morning Ben decides that he not only wants to keep the baby but also attempt a relationship with her and her previous children – but this is more a whim than a well-thought-out decision; nearing 40, Ben thinks finally settling down is what he wants, or at least what he's supposed to want.
At the same time, he sends Abigail mixed signals, to some extent paving the way for his eventual flight. Both engage in a vicious circle wherein she lowers her guard, and he lets her down; he returns with his tail between his legs, and she forgives him and takes him back.
Ricci remains as capable and effective as she was when she debuted 30 years ago, and 10TWSDBWBU is a worthy addition to her eclectic filmography. That said, it's Linklater who elevates his game to a whole new level; his acting is so subtle you don't fully realize everything that's bubbling under the surface while watching the film – it's only as I write this that I reflect on the nuances of his performance.
Ben could very well be a sociopath (and I think one character actually hints at that possibility). His boyish charm and rakish demeanor easily ingratiate him with Abigail and her family, and his immaturity and inexperience are the perfect excuse for his blunders – all the while underhandedly using Abigail's pregnancy, especially the deadline for an abortion, as some kind of ever-present ultimatum, to manipulate her.
Writer/director Galt Niederhoffer doesn't expect us to like or dislike Ben; he leaves a small margin of ambiguity, enough for us to exercise our own judgment (though Linklater can convincingly turn the insanity all the way to 11, as he does in the Amazon Prime series Tell Me Your Secrets – another showcase for his mastery of the slow burn). Finally, 10TWSDBWBU (movies about lists are invariably bad, but in this case the title is misleading) is honest about what happens after happily ever after, and that alone makes it worth watching.