I think Agatha Christie realized that Third Girl didn't quite work, so she retreated to more familiar territory with her next novel. Endless Night includes echoes of its era, with a girl I could easily see as a flower child, avant-garde architecture, and a more explicit linkage of sex and murder than we usually see from Christie, but with few references to cultural touchstones, it could almost take place any time between the late 1950s and the advent of the cell phone.
Mike Rogers isn't exactly a drifter - he's more of an opportunistic small-time adventurer who moves around Western Europe chasing short term jobs. He's been a fruit picker, a waiter, a stable worker, and a chauffeur, and it's while driving a rich couple to their almost completed house that he meets Santonix. The architect is young but dying, and believes he only has time to design one or two more houses; Mike is poor but appreciates Santonix's work. Perhaps it's a bit too coincidental that Mike soon meets an heiress by an allegedly cursed plot of land which would be perfect for one of Santonix's houses. Maybe we shouldn't believe that Ellie's paid companion is a gorgeous young woman only interested in allowing her charge to escape the restraints of her avaricious family. Christie, though, makes this set-up plausible, and since Mike is a non-omnicient narrator, we might expect kinks in the timeline or extra bits of improbability, chalking them up to the imperfections of human memory.
Mike and Ellie marry, move into the dream house Santonix designed, and immerse themselves in village society. They're joined by Greta, who's been fired by Ellie's family and, much to Mike's chagrin, continues to manage Ellie's life. Since this is a Christie novel, there is a death - Ellie's - and a death that looks like, but can't be, an accident. Mike is the natural suspect, but he was also twenty miles away at an auction with a pillar of the local community. Greta is innocent as well - she was shopping in London. No one else had a motive, and the only believable person to have an opportunity has disappeared. So who killed Ellie, and how? The solution is one of Christie's more surprising twists.