The Lonely House is a 1920 mystery detective novel by the British author Marie Belloc Lowndes.

[1] It features the French police detective Hercules Popeau, a near exact contemporary creation as Agatha Christie's Belgian private detective Hercule Poirot.

[3] Hercules Popeau appeared in several short stories and another novel One of Those Ways, but never enjoyed the same levels of popularity as Poirot.

Lily Fairfied, an heiress, goes to stay with her aunt at a house in Monte Carlo.

She finds the family impoverished and when a body is discovered nearby, she turns to Hercules Popeau for assistance.