Patricia Routledge returns for six episodes of the drama series about the Lancashire detective.
A widow calls on the Wainthropp Detective Agency to investigate the theft of a book written by her late husband.
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Dominic Monaghan throws some light on Patricia Routledge's case in Hetty Wainthropp Investigates

Hetty Wainthropp Investigates 9.30pm BBC1
Patricia Routledge returns for a welcome fourth series as the private eye with a fine eye for a floral frock. Hetty and husband Robert have scarcely unpacked after their Australian jaunt and she's already telling her sidekick Geoffrey that she's raring to go. With his girlfriend Janet much in evidence, the detective work seems to be growing into a kind of family business. It's appropriate, for Hetty's main aim is not so much solving mysteries as helping people.
This week her chief concern is a widow, played by Lesley Dunlop, whose husband died at the Cumbrian iron-ore mine where he worked. Hetty's attempts to recover his missing journal dig up some festering suspicions among the miners over valuable minerals.