The story unfolds with Matthew Stewart digging a grave for Grace Trevethyn's husband, John (played by Bill Weston), who has tragically taken his own life. Now a widow, Grace (Brenda Blethyn) finds her life turned upside down when she discovers that John's financial mismanagement has led to overwhelming debt, threatening her beloved estate and possessions with repossession. The burden of these debts is compounded by the incessant arrival of threatening letters from eager banks and credit card companies, with the local postman, Bill Hallet, delivering grim updates to her doorstep.
Amidst her turmoil, Dr. Martin Bamford (Martin Clunes), a friend of the couple, reveals that the community was aware of John's dubious financial dealings yet chose to turn a blind eye. As Grace struggles to comprehend the extent of John's failures, she learns about a secret Swedish account that holds no funds, leaving her with no viable assets or income. Forced to surrender her high-tech lawnmower to repo men led by Johnny Bamford and confronted with the need to fire her gardener Matthew (Craig Ferguson), with whom she shares a close bond, her situation becomes increasingly dire.
In a twist of fate, she discovers that Matthew has been struggling to cultivate marijuana plants due to poor sunlight conditions. Moved by her decision to let him go, Grace offers him space in her greenhouse for his plants. As they flourish beyond expectations, she innocently inquires about their value, only to learn that these plants are illegal and highly lucrative. An idea sparks within Grace that could potentially save her from losing everything to the bank—mass-producing marijuana.
However, not everyone is onboard; Matthew's girlfriend, Nicky Tristan (Valerie Edmond), a dedicated fishing boat captain, urges him to focus on settling down and taking responsibility rather than embarking on this risky venture. While some locals suspect illicit activities, they compassionately choose to overlook Grace's strange nighttime glow from her conservatory, likening it to an intriguing spectacle.
As the operation escalates, someone must venture to London to secure buyers for the harvest. Although Matthew, a cannabis user in his thirties, seems the obvious choice, Grace insists on taking the trip herself to ensure her gardener remains by Nicky's side, who is secretly pregnant. Her misadventures in the underbelly of London drug culture are both comedic and chaotic, and just when it seems she might fail, she encounters the charming Jacques Chevalier (Tchéky Karyo), who cunningly pulls her into a deal.
The growing operation inevitably attracts the attention of law enforcement, primarily due to the unmistakable lights emanating from Grace's greenhouse. Local officer Sgt. Alfred Mabely (Ken Campbell) initially dismisses the situation, believing her cultivation to be for personal use, only to realize the size of the operation demands an investigation. Grace's predicament becomes dire as she faces the necessity to destroy her stash. However, in a bizarre turn of events, the remaining moisture in the plants releases a cloud of smoke that enraptures everyone present—Grace, Matthew, visiting socialites, and even Mabely, all finding themselves joyously caught up in a euphoric revelry.
In an unexpected resolution, Grace channels her wild experience into a book titled The Joint Venture, which captivates readers and skyrockets to bestseller status, lifting her from financial despair. As the townspeople tune in to see her receive accolades for her work, it becomes clear that Grace and Jacques have formed a personal connection, with him having left his life in London to embrace hers, vehemently denying any past involvement in the drug trade.
