Forty three years ago, pretty, young and pregnant Rosie Jones (Emilia Fox) is travelling alone by train through the peaceful English countryside. While she quietly peruses her copy of Country Life - her eyes alighting on a feature entitled ‘A Perfect Home for a Perfect Family’ - a guard notices blood oozing from a large, ornate leather trunk, forming a puddle at his feet. The trunk bears her name Rosemary Jones.

Horrified, the guard calls the police at the next station, where Rosie is arrested: two dismembered bodies were found in her trunk. When interrogated, she explains that she recently discovered her husband was about to run away with his mistress, saying sweetly, ‘I couldn’t very well stand idly by, now, could I?’ Rosie is charged with manslaughter and is sent to a detention unit for the criminally insane.

Now in the present day, we meet Walter Goodfellow (Rowan Atkinson), the dithering but well meaning vicar. As he finishes his sermon in the idyllic English country village of Little Wallop, he is greeted by nosy old Mrs. Parker (Liz Smith) who snidely comments on the repeated absence of Walter’s wife Gloria (Kristin Scott Thomas) from church.

Meanwhile, Gloria is trying to have a lie-in but cannot ignore the frenzied yapping of a neighbour’s obnoxious dog. As she sleepily peers from her window, she sees a van in the driveway rocking violently and rushes down to find her teenage daughter Holly (Tamsin Egerton) with the latest boyfriend, wearing little more than a rapidly fading smile. Gloria marches Holly back inside the house in a state of semi-undress much to the glee of Mrs. Parker who has witnessed the whole unfortunate scene.

When Walter wearily returns home he is deep in thought, failing to notice that anything is amiss, and wanders off to his study to work on the sermon he plans to deliver at a forthcoming religious convention.

After berating Holly, Gloria leaves for her golf lesson with Lance (Patrick Swayze), her perma-tanned, sleazy American golf instructor. Lance is a hands-on double-entendre kind of guy: it’s very clear he has eyes for Gloria. Gloria can’t deny, though, that the feeling might be mutual.

The following morning, Gloria, deprived of sleep, drops her young son Petey (Toby Parkes) at school. When he sees the gang of bullies gathered at the school gates, he is clearly apprehensive, and as a delaying tactic asks his mother about her own childhood. Gloria tells him, not for the first time, that she was brought up in an orphanage run by nuns.

Returning to the Vicarage, Gloria and Walter find that the new live-in housekeeper they have engaged has sent some of her effects on ahead of her arrival: most notably, a large, ornate and very familiar leather trunk. This time, though, it bears the name Grace Hawkins.

Gloria’s parting shot to her husband is to remind him to pick up their son from school. True to form, Walter, lost in his sermon, forgets his fatherly duty, and as school ends for the day, the bullies set their sights on Petey. Out of nowhere, an elderly woman appears and helps him to his feet.

When Gloria returns home she panics when there is no sign of her son. She rushes to find Walter and together they race to the school, which is now closed. Convinced he has been kidnapped they drive around the locality and amazingly find him with a strange woman walking in a nearby field. Gloria is beside herself, but the lady calmly introduces herself, this is Grace Hawkins (Maggie Smith), their new housekeeper.

When they all return home, Grace meets the family, and is shown to her beautifully prepared room. “A perfect home for a perfect family,” she murmurs. Later that evening, the dog next door is still barking, and Holly introduces Gloria and Grace to her latest beau, David.

With Grace now ensconced in the family home, Gloria doesn’t waste any time racing off into the arms of golf instructor Lance. She’s desperate for a break from family life, and Lance believes he has the perfect solution to her problem: he plans to spirit her away to Mexico as soon as he can organize the trip. When Lance drops Gloria home, he secretly waits outside in the hopes of catching a glimpse of the nubile Holly as she undresses in her upstairs bedroom.

That night Gloria prays for a peaceful night’s sleep. At first the dog’s barking still keeps her awake. Then she hears a yelp, and then silence. Puzzled, she snuggles down again but soon hears the sound of someone coming into her house, up her stairs, and into the spare room - Grace’s room! Miraculously she sleeps in late the next morning. When she comes downstairs she finds Grace has prepared a magnificent family breakfast. This is too good to be true!

After breakfast, the family head off to Walter’s charity football match in which he is playing goal keeper, much to Holly and Petey’s embarrassment. The game doesn’t go well for Walter, but he cheerfully perseveres until he is taken off the pitch having sustained a fast ball to the solar plexus! As he rests on the clubhouse steps, Gloria panics when she spots Lance in the crowd; then she sees him chatting casually to Walter. When Gloria finally corners Lance she confronts him about taking chances, but he soothes her with talk of Mexico.

The following morning, Gloria notices Mr. Brown (James Booth) putting up signs for his missing dog. At school, while Gloria walks Petey into the building, to allay the bullies, Grace answers Gloria’s forgotten phone. It’s Lance giving her a graphic description of what he’d like to do to her on that Mexican beach! But when he discovers he’s talking dirty to the wrong woman, he cuts the conversation short. When Gloria returns to the car, and Lance calls again, she claims it’s her friend Lillian. But Grace knows otherwise.

Later, Grace asks Gloria about her and Walter - how they met. Gloria’s fond memories sour when she thinks how things have changed: “Why, when they find God,” she asks wistfully, “do they lose their sense of humour?” Gloria makes an excuse and sneaks out to meet Lance, but the all-seeing, all-knowing Grace is not fooled. She heads across to the church and interrupts Walter as he works on his sermon. To his amazement Grace tell him a religious joke and encourages him to introduce a little humour into his sermons from now on.

Meanwhile, Gloria and Lance frantically undress in a romantic hideaway on the beach, but when Gloria sees his idea of a sexy surprise she flees, unable to cope or to take him seriously.

That same afternoon, Grace leaves Walter at his desk, searching the internet for religious jokes. Grace then strolls out to pick Petey up from school. On arrival, she takes a pair of wire cutters out of her bag. After school, moments later, the school bullies’ pursuit of Petey is short-lived when they mysteriously lose control of their bikes and crash into a hedge.

Gloria arrives home and finds her family transformed. Holly is cooking for Grant, her latest boyfriend, and Walter tries out a few new religious jokes. The once-shy Petey rushes in with new found confidence and excitedly recounts the glorious incident with the bullies and his escape.

Later that night, Grace invites Gloria into her bedroom, and nostalgically shows her photos from her past. Grace reveals that her former husband was unfaithful and Gloria admits to her own dalliance, due to her unsatisfactory love life: but Grace is confident - “He’ll find his way back to you.”

In the middle of the night, Mr. Brown is still looking for his dog. When he catches Grace preparing to bury the dog’s body, he soon becomes her next victim. The next day Grace tells Gloria that Mr Brown is on holiday ‘Down Under’.

That morning, when Gloria and Grace are out together having tea, Lance appears and tells her he has the tickets for Mexico. Gloria falters momentarily but then agrees to join him on the trip. She introduces Lance to Grace who instantly, recognises his voice and soon puts two and two together.

Grace appears at the church once more and in no uncertain terms, reminds Walter that the Bible is full of passages about passion and sex. Finally enlightened, he hurries to join Gloria in the marital bed, and tenderly fulfils her womanly needs. Meanwhile, downstairs, Grace spots Lance outside the house filming Holly and smacks him over the head with an iron. The next morning, a newly confident Walter heads off to his convention, and Gloria is stunned when he puts off the ubiquitous Mrs. Parker who tries to pester him yet again about the church flower committee.

Gloria sneaks out to meet Lance to tell him she will not be going with him, and is puzzled when he doesn’t appear.

Back at the house, Holly has caught a glimpse of a news report discussing the release of the notorious Rosie Jones from prison and runs to her mother to deliver the chilling news that it appears Grace is a serial killer. Pennies begin to drop and together they sneak in to Grace’s room to examine the mysterious trunk. But Grace catches them and makes a surprising and disturbing confession to Gloria which will reverberate throughout the family.

Despite her shock, Gloria berates Grace about solving problems by murdering the people who cause her trouble, but Grace tries to justify her actions by showing Gloria Lance’s tape of Holly. Gloria still refuses to help Grace dump his body in the pond. Later, Holly watches the awesome sight of Grace disposing of his body, all by herself.

Meanwhile, at the convention, Walter delivers his humorous sermon to rapturous applause. The subject of the sermon: God’s Mysterious Ways.

When Mrs. Parker pops in the next morning, Grace, Gloria and Holly are convinced that she knows everything and is about to spill the beans. Grace sneaks up behind her, armed with a large skillet, but in the nick of time Gloria realises that Mrs Parker is only there to talk about the flower arranging committee. But she is too late, the shock is too much for poor Mrs. Parker and she suffers a heart attack, and drops dead at their feet.

Walter carries out the funeral service for Mrs. Parker, and that night Gloria catches Grace packing to leave. Gloria thanks her for saving her marriage and her family and Grace promises to keep in touch.

All seems well, until two council workmen turn up with plans to drain the infamous pond.

There’s only one solution to a problem like that...