Socially awkward Muriel Heslop is often the butt of jokes from her superficial and self-centered friends, including Tania, Cheryl, Janine, and Nicole. While she navigates her mundane life in the uninspiring beach town of Porpoise Spit, she often escapes into a world of fantasies filled with ABBA songs, dreaming of a luxurious wedding that will whisk her away from her oppressive family life, particularly her domineering father, Bill Hunter, a corrupt politician who regularly belittles the entire family.
During Tania and Chook's wedding, Muriel’s daydreams are shattered when she catches Nicole in a compromising position with Chook. The situation escalates when Dianne, a department store detective, wrongly accuses Muriel of stealing the dress she wears, leading to a public humiliation where the police escort her out of the reception.
Just as things seem to spiral out of control, Bill's rumored mistress, Deidre Chambers, invites Muriel into her multilevel marketing scheme. However, Muriel's so-called friends officially sever ties with her when they clarify she isn't welcome on an island holiday. Her mother, Rachel Griffiths, unexpectedly provides financial support by signing a blank cheque for her cosmetics business venture. Instead, Muriel takes this opportunity to withdraw a significant amount of cash, $12,000, and follows her friends to the island.
In a twist of fate, she reconnects with Rhonda Epinstall, a long-lost high school friend. Their friendship blossoms as Rhonda gleefully reveals to Tania the gossip about Nicole and Chook. Upon Muriel's return home, her mother confronts her about the stolen money, leading Muriel to flee to Sydney, where she shares a flat with Rhonda and adopts a new identity—Mariel. In Sydney, she finds a job at a video store and briefly dates an endearing, albeit awkward, man named Brice Nobes.
However, tragedy strikes when Rhonda suffers a fall that reveals she has a cancerous tumor in her spine, leading to multiple surgeries and foreseeable paralysis. As Muriel learns that her father's corrupt practices have come to light, she makes a pledge to take care of Rhonda and vows never to return to Porpoise Spit. Amidst Rhonda's health crisis, Muriel concocts a scheme to receive special treatment at bridal shops, indulging her obsession with extravagant weddings. Eventually, tensions mount as Rhonda confronts Muriel about her fixation, causing a strain in their friendship.
In a desperate act, Muriel agrees to marry South African swimmer David Van Arkle, receiving a hefty payment for her assistance. The grand wedding in Sydney is a spectacle; however, it serves as a backdrop for heartbreak rather than joy. Muriel's old friends are her bridesmaids, but Rhonda, feeling betrayed, chooses not to participate, while Muriel's father openly flaunts his affair with Deidre, oblivious to the chaos brewing.
The story takes a dark turn when Muriel's mother tragically dies by apparent suicide after a distressing incident involving shoplifting, as revealed to Muriel by her sister Joanie. Amidst the turmoil, David finally expresses his disinterest in Muriel, and at the funeral, they share an intimate moment that culminates in them consummating their marriage. However, Muriel comes to a stark realization that their union lacks love and honesty, leading her to end the marriage for her own sake.
Faced with her father's request to help raise her younger siblings, Muriel, emboldened by her mother's passing and her newfound voice, decides to confront him. She offers him a portion of her wedding money, demanding he cease his verbal abuse of her siblings. In a bold move, Muriel visits Rhonda's home to reclaim her independence and makes a pact to help her best friend move back to Sydney, leaving behind the toxic memories of Porpoise Spit. Together, they head to the airport, ready to embark on a brighter future full of hope and possibilities.
