It's the week of Dermot's fiancee Maria Nicholson's hen party, and Agnes Brown and her friend Winnie are excited about going... until daughter Cathy tells them they are not invited because they are too old. Undeterred, Mrs Brown decides she will crash the event and show the girls that she and Winnie are not too old to party. All she has to do is find out where the party is being held. Agnes thinks her luck is in when son Dermot tells her that the girls are meeting at local pub Foleys. But when Agnes bursts into the pub in full hen-party attire, it becomes clear that Dermot may have misinformed her.
That night, Agnes Brown mistakenly invites Maria's snooty mother Mrs Nicholson to dinner. Finding out that Mrs Nicholson is so sophisticated that she has two toilets in her house, Agnes has one fitted in an understairs cupboard in order to impress her guest. Despite this, and despite drafting in Rory's cordon bleu friend Dino to cook dinner, on the night Mrs Brown is incredibly nervous and turns to drink. With Mrs Brown increasingly desperate to delight Mrs Nicholson, the evening descends into chaos as the drink takes hold, the aftermath of Agnes's hen party mistake plays out, and Grandad decides to go to the toilet in the wrong cupboard.