The story is basically about a young woman whose parents have meddled in her life. The narrator, Lorna, tries to make the best of things by criticizing others. For example, she did not test into the grammar school so she talks about the mess there and the bad habits the kids pick up.
The refrain from the title, "you should have seen the mess" is repeated in this way, to discount things the narrator cannot be a part of or doesn’t understand. Mess is used as an excuse to leave things. For example, Lorna feels uncomfortable in her first job, so she decides to leave because of the rickety furniture.
After quitting her job, Lorna gets a new job at a neater and more modern facility, Lowe's Chemical Co. She makes friends with the Darby's, a doctor family who have money but a messy and unkempt house. Lorna is surprised that she likes them despite the mess. The Darby's introduce her to a chemist assistant whom she dates, but he was an orphan and only bathes once a week because that's the only time he has hot water. They then introduce her to an artist, Willy Morley, who also has an unhygienic house. They date, but he does not make advances to her and does not even paint her portrait, so Lorna decides that she cannot sink so low as to marry him.
"You Should Have Seen the Mess" is about a 17-year-old named Lorna who has graduated with a degree in typing and who works in a chemical company. She is obsessed with cleanliness and a kind of superficial presentability, and she speaks about her joy at attending a secondary modern school instead of the grammar school at which she might have gotten a better education, because the secondary modern school is, in her words, more "hygienic."
She previously worked at a solicitor's office but quit because of the cracked tea cups and the generally run-down appearance of the place. Lorna's mother, Mrs. Merrifield, keeps their council flat spotless and expects the same of other people. Though Lorna is gifted at English, she won't consider taking a job at a publishing house because the company's offices are messy, and she settles for a job at a chemical concern because it is modern and has the latest models of typewriters.
Lorna applies the same principles to dating. She befriends a doctor and his wife named Dr. and Mrs. Darby, though they are messier than she would like, and they introduce her to a good-looking chemist's assistant. She eventually tires of him because he is poor and won't be able to provide her with the "extras" she likes. She then dates an artist named Willy Morley. As he wears a dark shirt, she can't tell immediately whether he's clean. Though Willy cares for her, has money, and enjoys her company, she rejects him in the end because his place is too messy and "it would break my heart to sink so low."
Lorna is an obsessive person whose evaluation of the world around her is clearly distorted. Her obsession with cleanliness and with presenting a kind of middle-class respectability stems in part from her (and her mother's) insecurity. They live in a council flat, which is public housing in Britain. They want to be socially acceptable by keeping their flat spotlessly clean. Lorna's insecurity over her class status makes her unable to relate well to others and to find happiness.