"I'm a snake that's been carrying around my old skin for too long." This line, delivered by Keira Knightley (in an American accent that's rather good, actually) as Megan Burch is the summation of what Lynn Shelton's Laggies (alternately known as Say When in Britain) represents, particularly for what will later be known as Millennial Syndrome.
Considering the intense subject matters of Lynn Shelton's prior films, including Your Sister's Sister and Touchy Feely, it comes as no surprise that Laggies wouldn't hold back in its exploration of the utter stagnation that transpires in the ten years you spend recovering from the ease and promise of graduating from high school. Megan only begins to realize in her late twenties that she's been in a state of atrophy ever since prom night when she and her other three best friends broke into the school pool and jumped in together.
To her latent dismay, she is currently serving as a bridesmaid for one of her "best friends," Allison (Ellie Kemper), who treats her like a crazy person for her irreverent jocularity (e.g. titty twisting a Buddha statue's nipples). Feeling imprisoned by her group of friends, it seems that the one person Megan can be herself around is her boyfriend, Anthony (Mark Webber), who is also in his own unspoken state of degeneration. In spite of working as a sign holder for her father's (played by Jeff Garlin of Curb Your Enthusiasm) tax company, Megan manages to hold her emotions in check until she sees her father getting a hand job from Allison's mother in the courtyard area at the wedding.
It is at this point that she flees the scene all too readily (especially since her boyfriend just attempted to propose to her at someone else's wedding, which is essentially the lamest thing ever), finding herself at a grocery store where high schooler Annika (Chloe Grace Moretz) and her friends ask Megan if she can buy them some alcohol. Sharing some sort of instant "laggies" affinity, Megan ends up hanging out after giving them the goods, joining them in toilet papering Junior's (Daniel Zovatto)--Annika's best friend and love interest--dad's house.
The simple pleasures of enjoying the same things she used to as a youth make Megan hesitant to return to reality, where she must deal with answering Anthony's question about marriage. Although she agrees to elope with him in Vegas, Megan stalls by saying she's going to a career counseling seminar on Orcas Island for the week. And while she did originally intend to go, a call from Annika begging her to come to the school to pretend to be her mother for a parent/teacher conference changes her plans for the rest of the week, and, ultimately for the rest of her life.
Through seeing the same mistakes and insecurities that Annika has when Megan was once her age, she is able to gain the courage to finally have some inkling of what she wants from life--tracing back to the more recent adage: I don't know what I want, but I know it's not this.