Agnes Brown is concerned that her family are keeping too many secrets. Her daughter Cathy won't let her meet her new boyfriend. Her elder son Mark won't reveal why his feisty wife Betty has thrown him out. And her younger son Dermot has lied to his fiancee Maria Nicholson's well-to-do parents about the status of his job.
When Betty and Mrs Brown meet to discuss Mark's problem, they unsurprisingly come to blows. Unfortunately their row happens in front of Dermot's posh future mother-in-law Mrs Nicholson, who has come round to discuss arrangements for her daughter's wedding. Horrified by what she witnesses, Mrs Nicholson decides that the Brown family are so rough she'll try to stop them attending the wedding.
Then Mrs Brown spots Betty with another man, and concludes that Betty and Mark have fallen out because Betty is having an affair. But when she finally confronts Betty, Agnes is confounded to discover that Mark is the one who has something to hide: a guilty secret for which Agnes has only herself to blame.