When a young pregnant woman named Rosie Jones (Emilia Fox) boards a train, her enormous trunk starts leaking blood in the luggage compartment.
Questioned by the police about the dead bodies inside, Rosie calmly reveals they are her unfaithful husband and his mistress. Convicted of manslaughter, she is imprisoned in a unit for the criminally insane due to diminished responsibility.
Forty-three years later, Walter Goodfellow (Rowan Atkinson), the village vicar of Little Wallop, is very busy writing the perfect sermon for a convention. He's completely oblivious to his family's problems: his wife, Gloria (Kristin Scott Thomas), has unfulfilled emotional/sexual needs and starts an affair with her golf instructor, Lance (Patrick Swayze); his daughter, Holly (Tamsin Egerton), has a growing sex drive and physical maturity and constantly changes boyfriends without any reason; and his son, Petey (Toby Parkes), has been a victim of bullying at school for quite some time.
New housekeeper, Grace Hawkins (Maggie Smith), becomes involved in their lives, learning about their problems: neighbour Mr. Brown's Jack Russell terrier Clarence barks non-stop, preventing Gloria from sleeping; Petey has bullies; and Gloria has an affair with Lance.
Grace sets out to solve the problems in her own way by killing Clarence as well as Mr. Brown, sabotaging the brakes on the bullies' bicycles which injures one of them and killing Lance with a flat iron outside the house for videotaping Holly undressing one night.
As Walter prepares the sermon for the conference, Grace suggests adding humour. Also, seeing he has let his relationship slide due to his devotion to God, she shows him he can love his wife and God by looking at the erotic references in the Song of Solomon. As the problems in the household seem to gradually clear, Walter leaves for his convention.
Gloria and Holly see Grace's photo on the news, showing her release and previous offences, and they begin to realise what she's done. It is revealed that 'Grace' is Gloria's long-lost mother Rosie Jones, who's come to meet her. After briefly processing the flood of information, Gloria asserts that when having a problem with someone, one cannot just kill them.
Grace mentions this is the point she and her doctors could never agree on. Despite their disagreements, Gloria tries to help Grace with Lance's body, but cannot handle it. Over a cup of tea, the three girls decide not to tell Walter or Petey any of what has happened.
Nagging congregant Mrs. Parker (Liz Smith) visits to discuss the problem of the "church's flower arranging committee". Grace, erroneously believing Mrs. Parker is about to turn them in for her crimes, attempts to hit her with a frying pan but Gloria stops her. Mrs. Parker, shocked, has a heart attack and dies. Walter returns from the convention just then and sees Mrs. Parker's body, but not realizing she is dead. Soon after, Grace leaves the family when order is seemingly restored among them.
Walter then talks to Bob and Ted, the waterworks employees, who say there is too much algae and the vicar's pond needs to be drained. Remembering Grace's victims' bodies are in the pond, Gloria, with a strained smile, offers them some tea.
The film ends with an underwater shot depicting the bodies that had been placed in the pond, including the recently added Bob and Ted.
Rowan Atkinson as Reverend Walter Goodfellow
Kristin Scott Thomas as Gloria Goodfellow
Maggie Smith as Rosie Jones/Grace Hawkins
Emilia Fox as Young Rosie Jones
Patrick Swayze as Lance
Tamsin Egerton as Holly Goodfellow
Toby Parkes as Petey Goodfellow
Liz Smith as Mrs. Parker
James Booth as Mr. Brown
Patrick Monckton as Bob
Rowley Irlam as Ted
Vivienne Moore as Mrs. Martin
Murray McArthur as the Vicar's Convention Master of Ceremonies
Roger Hammond as The Judge