In short the movie is about 2 couple who have been in a long relationship in the past who are trying to move on, Allison (Cobie) invites his ex-boyfriend Adam (Justin) to attend her wedding with her new boyfriend (or future husband), things down the line start getting weird as Adam starts to remember the past and the relationship he had with Allison, and by the end of the movie he tries his best to get her back, but ends up screwing it all up for good.
*Note: In real life situation you wouldn't dare attend your ex-girls wedding nor would she even invite you, i think it could have been more sensible if it was centered around for example an engagement party of some kind, because that would be your very last chance to turn things around (if you can), not at an actual wedding where everything is already pre-prepared, so i think in that context the film could have taken a different approach, i was happy though on how well Allison (Cobie) and Adam (Justin) acted with their face impression, mimics and essence of the characters.
*Note: In real life situation you wouldn't dare attend your ex-girls wedding nor would she even invite you, i think it could have been more sensible if it was centered around for example an engagement party of some kind, because that would be your very last chance to turn things around (if you can), not at an actual wedding where everything is already pre-prepared, so i think in that context the film could have taken a different approach, i was happy though on how well Allison (Cobie) and Adam (Justin) acted with their face impression, mimics and essence of the characters.