Over Sunday lunch, the Collins family, who reside on the 13th floor of a London tower block, discuss the forthcoming conversion to decimal currency. The male members of the family are confident about the change but the mother, daughter and granny are apprehensive. Granny Collins is particularly anxious about the new system and wakes up screaming from her Sunday afternoon nap during a surreal nightmare in which decimal points are fired at her from all sides. The fashion-conscious teenage daughter, Sandra, is less concerned and busies herself trying on a series of fashionable wigs.

At school Peter Collins' maths teacher explains the how the decimal system works and urges the students to pass on their knowledge to their elders who might find it more difficult to make the change.

Mrs Collins, addressing her family through the kitchen hatch, relays her concern that the new system might make it easier for shop-owners to put the prices up, but Mr Collins assures her that there are readily available decimal conversion tables which will prevent this. Granny Collins' opposition to the system is steadfast, however, and she threatens to go on a hunger strike in protest.

D-Day arrives and the Collinses are having breakfast together - everyone, that is, except Granny, who has locked herself in her bedroom. Mrs Collins wonders whether she should to go to the post office to convert her old money into new money. Her husband tells her that this is unnecessary since old money will still be in circulation.

Left alone in the flat, Granny Collins opens the door to the milkman who tries to explain the new coinage to her in simple terms, but only exacerbates her confusion. She starts to cry.

The other members of the family start using the new decimal system in their respective situations: Mrs Collins at the bakery and the grocer; Mr Collins at work in his office; Sandra Collins at the boutique and Peter at school. Sandra struggles the most and is unable to complete a decimal transaction with a customer.

Peter kindly gives his grandmother a lesson in decimalisation and, after much patience on his part, Granny Collins does finally 'get the point'. When they visit Sandra at the boutique, Granny surprises a group of confused young shoppers with her grasp of the new system.