“Mamma Mia!” is not so much a musical theater piece as it is a party.

Sure, a story unfolds about a mother and a daughter and her three potential fathers, and we will meet characters who are strong, some who are regretful and some who wonder about their lives.

Or as Christine Sherrill, who plays Donna, says, “It takes place on a Greek Island during a wedding (weekend) and chaos and ABBA ensue.”

“You got me at the word ABBA,” adds Katie Bates, who plays Donna’s daughter, Sophie.

From the moment you walk into the Fulton Theatre, where “Mamma Mia!” opens tonight, you will be escorted to Kalokain, the fictional Greek Island where Donna runs a tavern and where 20-year-old Sophie is marrying Sky.

“Everyone who comes to the theater is invited to the wedding,” Bates says. “It’s an immersive experience.”

Three months earlier, when she was planning the wedding, Sophie dreamed about her father walking her down the aisle, but she didn’t know who her father was.

“My mom has been everything and more,” Bates says. “She doesn’t talk about the past.”

Sophie finds her mother’s diary and discovers that her mother was seeing three different men around nine months before she was born. Any one of them could be dad.

“Maybe I could find out which one he is, so I send all three invitations,” Bates says. “And they all show up at once.”

“Donna is surprised when they show up, but she takes the journey,” Sherrill says. “It’s a chance to reignite friendships and passions. She is not a dainty flower. She prides herself on that.”

This will be Sherrill’s fourth production of “Mamma Mia!” and her third time playing Donna, including a long stint in Las Vegas.

“Groups of women would come together to see the show,” she says. “It portrays strong women in a joyful way. There is no shame in it. And, of course, ABBA sure is No. 1 for everybody.”

The songs in the show are loosely weaved together to help tell the story.

The musical includes such hits as “Super Trouper,” “Lay All Your Love on Me,” “Waterloo,” “Dancing Queen,” “Knowing Me, Knowing You,” “Take a Chance on Me,” “Thank You for the Music,” “Money, Money, Money,” “The Winner Takes It All,” “Voulez-Vous” and “SOS.”

In all, about 25 ABBA songs will be sung.

And with Marc Robin directing, you know there will be plenty of dancing, too.

“I keep telling people it’s not going to be like any ‘Mamma Mia!’ you’ve seen before.”

Sherrill says she owes her career to Robin.

Fifteen years ago, Sherrill was a teacher and a single mother. She’d gotten a scholarship to study at St. Xavier University in Chicago and had to perform to keep it.

Robin saw the performance and asked her to audition.

“He told me, make a choice. Leave teaching and go with the show.”

It seemed like a tough decision at the time, but Sherrill decided to go with the show.

“I never looked back,” she says with a laugh.

Audiences have loved the show since it opened in London’s West End in 1999. It’s the ninth-longest running Broadway show in history, playing for 14 years. In all, more than 60 million people have seen the production in one form or another.

The two biggest challenges in the show are all the singing and all the costumes changes.

“It’s physically taxing, and ABBA is a hard song,” Sherrill says. “But if you’re doing your job right, it’s kind of foolproof.”

The costumes, especially those at the end, are all about 1970s excess.

“The costumes changes have been choreographed to a T,” Bates says. “The wardrobe (people), the musicians, the crew — everyone — is hustling. Thank God it’s so much fun to do.”