The year is 1915. Our heroine is Helen McGill, a thirty-nine year old spinster who assists her brother Andrew in the running of his New England farm. Andrew is a literary man, however, and since his books have become successful, his attention to the farm and his appreciation of the work that Helen does have diminished considerably. Helen takes pride in her domestic accomplishments but she doesn't like being saddled with all the work and she doesn't like being taken for granted. Enter our unlikely hero. Roger Mifflin rolls into the farmyard in the ''Parnassus on Wheels'' of the title: a travelling bookshop. He's been travelling around the countryside evangelizing about literature but he's ready to pack it in to write a book of his own. He's seeking a buyer for the Parnassus and he thought that Andrew would be a likely candidate. In an uncharacteristically impulsive moment, Helen makes the purchase herself. She rationalizes that she's simply trying to prevent Andrew from doing the same and disappearing from the farm once and for all. But it soon becomes apparent that the seemingly settled Helen is out for a bit of adventure. In the travelling book trade, she finds it.