Ria Parkinson (Wendy Craig) is a bored housewife and mother. She spends her time daydreaming, and meets regularly with wealthy businessman Leonard to relieve the monotony.
Husband Ben, a dentist and avid butterfly collector (Geoffrey Palmer) is oblivious to it all, and her unemployed grown up sons, who both live at home (Nicholas Lyndhurst - who went on to better things, and Andrew Hall - who didn't) also have other things on their minds, especially girlfriends.
Leonard is back from New York and Ria has received a letter from him but before she can read it Ruby comes in so she hides the letter in a magazine. Adam goes off with the magazine and swaps it for one of his friend Guy's. The magazine gets left in the park and Ria raises eyebrows in her efforts to find it. Eventually Adam gets the letter back for her. Ben reads it but it is quite innocuous and Ben assumes it was written by an old school friend.
Six months have elapsed and Ria meets Leonard in the park. They hug, he says he loves her and wants to carry on with her. Meanwhile Ben is having lunch with a widowed friend Amanda. She seems to harbour similar thoughts towards Ben as Leonard does for Ria. Back home Ria cooks up another disastrous dessert but the family rally to support her and Ben says he is taking her to Paris for lunch.
Ben and Ria return from Paris to find the boys have had a party and somebody has left a tin of pot lying around. Ben blames Ria for being too liberal with their sons. She goes to the park. There is no Leonard,who is tearing up her photo as he sees their relationship will go nowhere,but she meets a pregnant Jeannie who envies her her secure marriage. Back home Ben makes the boys promise never to smoke pot again but to Ria it's just what she needs to help her relax.
The boys have the opportunity to buy a window cleaning business but Ria is still depressed and goes to church for guidance. When she comes out she finds Leonard has put his diary in her car . She phones him but a woman answers.Russell learns that he is a father. Ria now feels very old,appreciating that she is a grandmother,but Ben is unusually understanding towards her.
Russell visits Jeannie but feels depressed because she only wanted his sperm and wants the baby but not him. Ria is also depressed after yet another unsatisfactory meeting with Leonard. Ruby, who has been chatted up, is the only person pleased about anything. Leonard rings Ria but Ben answers. He says nothing but next day he catches a butterfly and lets it go, telling it to look after its mate.
Whilst the boys are happy to meet randy housewives on their window round, Ria is feeling old, needing her glasses to read the menu at lunch with a friend. Again she splits with Leonard, this time observed by Ben. She confides everything in Adam, who tells her Ben is lucky to have such a faithful wife. Meanwhile Ben takes a call from Amanda, who has returned from Spain.
Ben resists Amanda's advances and takes her back to the station. Jeannie tells Russell she wants him in their baby's life after all. Ria is feeling frustrated,unaware that Ben has given Amanda the brush-off and feeling taken for granted. She packs her bags ready to leave but when she sees Leonard in the park she cannot go through with it. She senses that Ben loves her after all and returns to him.