The first scene shows Matthew Stewart digging a grave for Grace Trevethyn's husband, John (Bill Weston), who has committed suicide. His now widow, Grace (Brenda Blethyn), an English woman living on a beautiful large estate, learns, upon his death, that John's debts had mounted to the point that her assets (home, car, garden) will soon be repossessed. The postman (Bill Hallet) keeps on bringing her more threatening letters from banks, credit cards... and he also tells all the townspeople about her problems. One of John and Grace's friends, Dr. Martin Bamford (Martin Clunes) tells her that everybody knew more or less that John was doing something fishy financially, but nobody wanted to interfere in his business. With desperation, Grace realises that all commercial schemes by her late husband were failures, and that they even had a secret account in Sweden, but with nothing in it.
With no assets of note and no source of income, she watches helplessly as the process begins, and when her high-tech state-of-the-art lawnmower is taken away by some movers (led by Johnny Bamford), she must fire her gardener, Matthew Stewart (Craig Ferguson), to whom she is close. When she stumbles upon the fact that Matthew is not having much success growing some weed plants in the fields of the local vicarage because of the lack of sunlight and heat, feeling awful about having fired him, she suggests that he move his plants to her greenhouse.
The plants begin to grow impressively, and Grace, quite innocently, asks Matthew what the plants are worth. The news that they are illegal marijuana plants that are quite valuable on the open market gives Grace an idea, one that might just save her from having her estate repossessed by the bank. Mass production of marijuana plants begins in Grace's greenhouse. Matthew's girlfriend, hard-working fishing boat captain Nicky Tristan (Valerie Edmond) wants no part of the plan. She wants Matthew to grow up, settle down and start taking responsibility for his own life. Some of the locals suspect what is going on, but, compassionately, stay out of Grace's way. In fact, some of them have the notion of watching the neon lights coming from the conservatory every night, as though it was some kind of show.
Of course, somebody will have to go to London to line up a buyer for the marijuana. Matthew, a marijuana user and a man in his thirties is the logical candidate, but unbeknownst to him, his girlfriend is pregnant. Knowing of the pregnancy, and not wanting Matthew to leave the side of his girl, Grace insists that she be the one to go to London. Her escapade into the lowest depths of the London drug subculture is hilarious, and at first it looks like she is going to fail. However, suave and savvy Jacques Chevalier (Tchéky Karyo, more handsome than ever) eventually contacts her. He smooth-talks her into doing business.
Authorities become aware of the drugs on her estate because of the flying saucer-like lights which the greenhouse now emits. Local police officer Sgt. Alfred Mabely (Ken Campbell) has let things be, as he thought she was cultivating just a small quanitity of weed for her own personal use and that she needed it to cope with all the debts and stress. But when he realises that it's a huge quantity, he has to get involved and investigate the matter.
Grace has little choice but to destroy her own inventory. However, as the marijuana plants are not completely dry, they produce such a huge quantity of smoke that Grace, Matthew, the posh visiting ladies (Phillida Law, Linda Kerr Scott, Denise Coffey) who didn't wait for Grace to appear to make themselves some tea, and even the police officers -including Mabely - get high. The scene leaves with everybody jumping, dancing and caressing happily in the garden while naked.
In a strange twist of fate, however, Grace writes a book describing her remarkable experience called The Joint Venture. The novel becomes a best seller, and, suddenly, her financial worries are over. She has to travel again to London to receive an award won by her novel, while all the townsfolk look at her on TV. Now we come to realise that Grace and Jacques are a couple, that he has left London for her small town and he strongly denies to some journalists that he ever got involved with any kind of drug.