Agnes Brown is concerned that her family are keeping too many secrets. Her daughter Cathy won't tell her anything about her new boyfriend. Her elder son Mark won't reveal why his wife Betty has thrown him out. And her younger son Dermot has a troublesome secret too. He's lied to his fiancee Maria's well-to-do parents about the status of his job in promotions. When Betty and Mrs. Brown meet to discuss the situation with Mark they unsurprisingly come to blows. Unfortunately this happens in front of Dermot's posh future mother-in-law Mrs Nicholson. Horrified by what she's seen, Mrs Nicholson determines that the Brown family shouldn't attend the wedding - and changes the venue from Dublin to Italy. Then Mrs. Brown spots Betty with another man and decides 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 with the guilty secret.