Photographer Nick Tyler and his wife Angie have just bought a house for them and their new-born son but want to let the basement flat to improve their finances. Nick's father Sam, recently made homeless, is keen to move in but, unbeknown to him and Nick, Angie's mother Daphne also arranges to take the flat to be the child's nanny. Daphne is refined and hypercritical. Sam is down to earth and unassuming. As co-tenants they will prove to be a very mismatched couple.
Sam settles in to the basement and proves himself a champion nappy changer, though he is less successful at winning over reluctant flatmate Daphne. She is also displeased to hear that her grandson will be called Joe and not Jeremy and will not be christened. Daphne hopes to get her vicar friend Gerald on her side and is appalled when he and Sam hit it off totally, leading to Gerald upbraiding her for being a snob.
Four weeks have passed and Daphne is still hostile to Sam, prompting Angie to threaten her with eviction unless she changes her attitude. When Daphne entertains her friend Elspeth to tea Angie forces her to include Sam in the party but things so very wrong when Elspeth discovers the dead snake Sam has brought home for his new hobby of taxidermy.
Daphne is dismayed that Angie is returning to work but cheered to get a dinner invitation from Penelope, a girl whom Angie hated at school but whom Daphne admires as she married the wealthy , boorish Nigel. Daphne is appalled to find that Sam has also been invited. However when Nigel tries to scam Daphne out of money for a fraudulent investment Sam puts aside his dislike of her to do the decent thing.
Angie, Nick and Joe are away for the weekend and, having eaten some condemned pastries, Sam reckons that if he feigns illness Daphne will be nice to him. He even persuades his zoo-keeper friend Wilf to pose as a doctor but when Daphne injures her toe Sam pretends to expose Wilf as a fraud to impress her. She is unusually grateful and gives him a reward - which proves to be just desserts.
Daphne has sold her house in Cheltenham and offers Sam some of the proceeds if he will move out. Sam counters by saying that he agrees - but only if he fails to make her laugh. They then spend a day together, in the pub, visiting Sam's wife's grave and at a restaurant. Next day Sam succeeds not only in making Daphne laugh but getting her to admit why she is so bitter.