Greene's main character, Henry, is a former branch bank manager who has retired early and spends his waking hours tending to the dahlias in his garden, secure in his propriety. But at his mother's funeral he meets a long-lost aunt who lures him progressively out of his comfort zone.
The novel starts slowly, with little obvious plot, but incrementally, we realize that Aunt Augusta is engaged in behavior that she justifies as "minor offenses that hurt no one" but which also draw the attention of the authorities on three continents.
Greene is at his best in the final chapters, when Augusta lures her cousin to a remote town in Paraguay where she is holed up with a former lover who has a questionable past and an even more questionable plan to get rich, and Henry has to decide how much excitement he really wants in his life.
If there is a weakness in the novel it is that for Henry the stakes are not that high. There are no revolutions to join or spy rings to disrupt, merely some minor-league (and a little major) smuggling. There is also a coincidence Greene asks us to believe, that a fellow passenger on the Orient Express would turn out to have a relative in remote Paraguay who has control over Henry's fate.
But for me at least, Greene pulls it all together in the end. His hero is not all that memorable, but if taking charge of one's fate is heroic, then Henry makes the grade.

Can’t understand why this is considered one of his best novels. I thought the first half was enjoyable enough, but the humor began to pall in the second. ( )

DNF

Enjoyable counterpoint if life philosophies ( )

At first one thinks Travels with my Aunt is a comic accounting of an author/narrator with an eccentric relative. It is: then it takes some peculiar and even murderous twists.
There is plenty of comedy, though, as we are set up for sudden shocks by the traveling companions, reticent retired bachelor banker, Henry Pulling and Augusta Bertram, a wildly uninhibited septuagenarian.
Much of the humor arises from Greene's descriptions: when describing a pair of American fellow-travelers serving tea, "One of them was raising a little bag, like a drowned animal, from his cup at the end of a cord. At that distressing sight I felt very far away from England ..." Or from Augusta's insouciance: " 'I can't call you Henry. It doesn't sound like a real name. Can I call you Smudge?'
'Why Smudge?'
' I had a dog once called Smudge. I used to talk to him a lot.' "
Much of the humor ends when Augusta's former lover, Nazi collaborator, Mr. Visconti enters into the picture.
Greene's writing never disappoints. This novel is no exception.