This image released by Amazon Prime Video shows Alison Brie in a scene from “Somebody I Used to Know.” (Scott Patrick Green/Amazon Prime Video via AP)

At least Julia Roberts was invited to the wedding.

I heard myself saying that out loud, rather indignantly, about halfway into “Somebody I Used to Know,” the new wedding-crashing rom-com directed by Dave Franco and starring real-life partner (and co-writer) Alison Brie, in which parallels to “My Best Friend’s Wedding” come early and often.

Roberts’ crafty Julianne, if you recall, hopped a plane to disrupt the wedding of the guy who got away years ago. But she was greeted with open arms at the airport, not only by the groom but by the blushing bride. Here, Brie’s Ally basically crashes her ex’s nuptial week — to the distress of the bride — creating a scenario which feels a lot meaner. Oh, she’s charming and all, but her mayhem is a lot more devious.

Of course, as these movies go, people end up as their better selves, eventually. But there’s a late, desperate move on Ally’s part in this often entertaining but also hectic and hit-or-miss story that’s so nasty, you just stop rooting for her — and wish Julianne would swoop in, 25 years later, and set her young doppelganger straight.

We first meet Ally, a Hollywood showrunner, as she’s wrapping up the third season of her reality TV show, “Dessert Island” (wherein top chefs compete to create desserts on an island, of course). Ally is great at getting people to admit private stuff on camera. But the show suddenly gets canceled.

Devastated, Ally goes home and cries with her cat, then consults her agent (an amusing Amy Sedaris), who suggests she stick with the dessert theme — maybe a show called “Dis-Hurt Locker,” involving Jeremy Renner and baklava? (Brie and Franco have lots of fun sending up reality TV. Which is much funnier than the cat on the plane with gastric distress.)

Speaking of that cat, Ally and her feline friend hop a plane to her hometown in Washington state, whereupon she walks in on her mom (Julie Haggerty, also amusing) having sex with her 3rd-grade teacher. Escaping to the local bar, she runs into her ex, Sean (a terrific Jay Ellis, both effortlessly charming and vulnerable).

They decide to get a quick bite, which leads to a full night of drinking, square-dancing, eating huge pretzels, guzzling melted cheese, puking from alcohol — you know, all those things you do when you run into your ex. At sunup, Ally suggests the two go home and get even more reacquainted, an offer Sean politely turns down.

Oops! Turns out, he neglected to mention he’s engaged! Ally doesn’t learn this until the next day when, popping by Sean’s family home and catching up with his lovely parents, the mother gives a toast to the soon-to-be newlyweds. Bride-to-be Cassidy (Kiersey Clemons) is ostensibly everything Ally is not, at least now — a free and rather innocent spirit, a punk rock musician, and young, very young.

Anyway, it’s too late, Ally! That’s what old friend Benny (Danny Pudi, in the sort-of Rupert Everett role) tells her, and it’s what anyone would tell her. But, like Roberts’ Julianne, Ally decides it’s not too late to nab the groom out of the jaws of matrimony.

The parallels keep coming — remember that karaoke scene, where Cameron Diaz’s tone-deaf (literally) bride is forced by Julianne to sing, but ends up charming the whole room? Here it’s Ally who is baited by the bride (understandably suspicious of Ally’s motives) into performing before a crowd, with similarly unexpected results.

Cassidy’s suspicions are clear. “You’re not going to pull some Julia Roberts ‘Best Friend’s Wedding’ type (stuff), are you?’ she asks. Why, no, Ally replies. But obviously she is. Soon, Ally’s taken on the role of wedding videographer.

It’s an edgier, much updated plot, in which Brie and Franco try to tie in themes of female empowerment and work-life balance along with serious difficulties each partner had to overcome earlier in their lives. There are also elements of bisexuality and public nudism. It’s a kitchen-sink approach.

Not everything works. Especially perplexing is a half-baked subplot involving one set of parents, and this is where the story lost me, making Ally seem not only selfish but actually rather cruel, and not as smart as we thought. We end up feeling sorry for virtually everyone else.

Ally will redeem herself in other ways, as did Julianne. But the messiness of the plot makes us nostalgic for the source inspiration. As we wind our way to a hectic conclusion, can you blame us for simply wanting to see Roberts put down her enormous 1997 flip phone (how DID that thing ever fit into an evening bag?) and dance with her dashing bestie, George, at the wedding? Check it out on YouTube. Some things don’t get old.

“Somebody I Used to Know,” an Amazon Studios release, has been rated R by the Motion Picture Association of America “for sexual content, graphic nudity, language throughout and brief drug use.“ Running time: 106 minutes. Two stars out of four.

MPAA definition of R: Restricted. Under 17 requires parent or adult guardian.