Rachel has always been the good girl while her best friend Darcy is the wild one. On the night of her 30th birthday, Rachel strays from the good girl path and ends up sleeping with Darcy's fiancé Dex, complicating everything.
The wedding is still on and neither Dex not Rachel are prepared to make any choices regarding their relationship. Rachel wants Dex but wants to stay friends with Darcy, and Dex thinks he has gone too far down the road with Darcy to call off the wedding, much as he can't get Rachel out of his head.
I recently read the book, and I have to say that Something Borrowed the movie was pretty faithful to the book, with only very slight variations - like Ethan, Rachel and Darcy's homeboy being more in the picture than he is in the book.
And I have to say that Ethan was pretty much the only person I liked in this whole movie. In the book, I sort of liked Rachel and Dex, but in the movie they both came off as completely spineless. Darcy was mean as ever but she wasn't fantastically mean, so it was rather bland. Bitchy, but bland.
I found the movie slow and not very funny save for Ethan, so I give it a C-