The plot of the book unfolds on two parallel levels, that of action and that of psychological drama.

The first presents a familiar image: The beloved Pacific Ocean, seen first from the depths of the Nautilus (in 20,000 Leagues Under the Sea), now unravels through the eyes of sailors on a sailing ship. In a way, revisiting it was a debt owed, both to that book and to The Children of Captain Grant, which did reach Australia but... did not see it in detail. Now, with a similar mission of exploration and rescue as the motivation, Verne found the opportunity to describe in detail both the islands of Oceania and Australia itself.

Meanwhile, Mrs. Brannigan waited, waited, and experienced the myriad psychodramas of a 'Penelope,' as well as the consecutive misfortunes that followed the 'one evil.' This is the second level of the book, which is highly emotional and in which Verne shows particular sensitivity.