A one-off special of mystery show Jonathan Creek is being filmed near Hemel Hempstead today, Monday.
Actress Sheridan Smith will be in the episode, which is being shot on the Gaddesden Estate, off Red Lion Lane, and Upperwood Farm Stud, off Gaddesden Row.
Sheridan recently played the part of Mrs Crumb in one-off BBC One show Mr Stink, which was also filmed in Hemel Hempstead and featured location shots of Marlowes and Gadebridge Park.
The family drama was one of the critical hits of the Christmas TV season.
Sheridan tweeted earlier today: “Filming in a field in Hemel Hempstead all day, thank God for thermals!”
Pink signs labelled ‘unit’, ‘base’ and ‘Loc 2’ have been posted on roads around the town, directing film crews towards their locations.
Filming in the area will be finished by the end of the day.