There is more to this than Timothy acting as an inept prompt for his local drama group's production of 'Hamlet. The episode is inspired by the play itself.
Mr Lumsden has gone away on a break and uncle Claud is in town. Mrs Lumsden always had a shine for her husband's brother. Much to Timothy's shock.
As for the play itself. Geoffrey (Douglas Hodge) is nervous playing Hamlet. Timothy's lack of concentration annoys Frank.
At least Timothy finds a friend in Valerie who is also in the play. Unfortunately he ends up insulting Valerie as he rushes to find out just who is in his mother's bedroom.
It is a bit of a bard farce and rather good. Once again Mrs Lumsden insults Timothy in front of Valerie.