The story revolves around his penniless niece Mary, Thorne's only blood relative, for whom he has great affection and is very protective.
Mary falls in love with a lovely young man named Frank, the heir to Greshamsbury Estate, and he with her. The only problem is Frank's mother would much rather he married a wealthy American in order to save their family from financial ruin.
There are plenty of other characters, like drunken railway millionaire Sir Roger Scatcherd (Ian McShane), and the snobbish Countess de Courcy, who get among proceedings and complicate matters, and what results is a great comedy of manners, within the confines of an everyday story.
"I was fiercely drawn to the challenge set by Trollope's ambiguity, to bring his wonderfully modern-seeming characters, who are neither all good or all bad, to the screen," explains Fellowes.
He was also drawn to Trollope's combination of comedy and generosity in his characters, and in particular to the straightforward nature of his hero.
"At the centre of them all, Doctor Thorne, played here so brilliantly by Tom Hollander, is probably one of the most uncomplicated heroic figures that Trollope ever wrote. But his heroism, if we may so define it, is of the modest and self-effacing type."
Along with Hollander, who last worked with Fellowes on Gosford Park, the cast includes Mad Men's Alison Brie, Stefanie Martini, Harry Richardson, and Rebecca Front.