Middle-aged businessman Edwin (Ted) Nugent keeps a flat in London where he entertains his mistress, Margo, on Tuesday nights. She complains that Ted's wife Betty is too nice: she'd find the situation easier if she hated her. She criticises the wallpaper, based on a purple grape motif. Ted explains that he has the same wallpaper in his bedroom. Margo is uncomfortable with this, so Ted suggests that she change it.
Ted drops Margo off near her home, next door to his, and goes for his regular swim - allegedly 36 lengths, in reality only enough to preserve the illusion, going home with his hair wet and smelling of chlorine. She tells him about a bargain-priced dry-cleaning service that she's discovered.
On Saturday, Ted and his friend Frank play golf, while Betty assembles Ted's clothes for dry-cleaning. A letter falls out of his jacket pocket into the electric fire, singing the edges. Betty looks for a clean envelope, and is about to reseal the letter when she sees that it's an order to a building firm concerning the installation of a bed in the flat. She examines Ted's keys, and fails to recognise two of them.
Betty drives round to the flat. The doorbell is labelled 'Miss W.Reilly', but there's no answer when she rings it. Ted's keys open the front door and let her into the flat. She is shocked by the wallpaper, and also finds a rumpled bed, a lacy handkerchief, a newspaper dated the previous Tuesday, and a blonde hairgrip. One of the building's other tenants tells her that Miss Reilly moved out three months earlier, and was replaced by a gentleman who was rarely there.
When Ted comes home, Betty deliberately unnerves him with innuendo, but says nothing more direct.
On Tuesday, Betty solicitously prepares everything for Ted's evening swim while hatching a plan of her own - but her babysitter rings to cancel the appointment. She rings around, finally alighting on Margo. Curious, Margo comes round and Betty tells her the whole story, saying that she plans to hide in the wardrobe with a large mallet and attack the woman, as she did in a similar situation five years earlier. The brunette Margo is relieved to hear that Betty thinks that Ted's mistress is blonde, and realises that she has no choice but to agree to babysit.
At work, Margo thanks her blonde secretary Julie for lending her her hairgrip, and meets Ted in the pub, telling him everything and insisting that her name not be mentioned. Unable to persuade her otherwise, Ted returns to the office and rings a friend.
Betty goes round to the flat and waits for Ted to appear. After a false alarm, she hears footsteps and hides in the cupboard, mallet in hand. She hears a woman's voice, breathless and expectant. When the couple enter the bedroom, Betty emerges from the wardrobe, locks the door and smashes the television set and record player while cursing Ted for his infidelity. She then opens the bedroom door...
...and finds Frank in bed with Julie, who leaps to her feet, slaps him and leaves. Betty is embarrassed and confused. Frank 'explains' that the flat is his, and that Ted arranged the redecoration - hence his name on the bill. Betty is hugely remorseful, offers a blank cheque to Frank for the damage, and then makes a different kind of offer by removing her clothes. It's Frank's turn to look bemused, but he succumbs to temptation.
Betty and Frank part, Betty insisting that their liaison was a one-off. She goes home and tells Margo that Ted is innocent. Ted, who has swum the full 36 lengths and is consequently exhausted, comes home to the warmest of welcomes.
Before giving the keys back to Ted, Frank has them copied for himself. He tells Ted that he and Betty had a quiet chat, then they went home. Ted calls him a true friend.
Ted and Margo meet up again in the flat - but when they move to the bedroom, they find Frank and Betty there already. Ted awkwardly introduces Frank to Margo.
The Nugents and Margo put their houses up for sale.
