S1·E8 The Actor and the Alibi

Mundon Mandeville, who runs a reviled repertoire theatre troupe, is murdered inside a windowless room. Everyone has an alibi. Father Brown is there as a favour to Flambeau to consult a 17-year-old into returning home to her worried parents who don't realise she has lied about her age in order to follow her dreams as an aspiring actress.