Complex sentence: In September 1957, she made her first professional stage appearance with the Old Vic Company, at the Royal Court Theatre, Liverpool, as Ophelia in "Hamlet", then her London debut in the same production at the Old Vic.
Split into simpler sentences: In September 1957, she made her first professional stage appearance with the Old Vic Company, at the Royal Court Theatre, Liverpool, as Ophelia in "Hamlet". Dench then made her London debut in the same production at the Old Vic.