Eventually every good series needs to be put to bed. Drawn to a close. Wound up. Killed. In spite of that [a:Burroughs|10885|Edgar Rice Burroughs|https://d202m5krfqbpi5.cloudfront.net/authors/1207155710p2/10885.jpg] is soldiering on with his stories from Barsoom. The first three books focussed on John Carter and his beloved Dejah Thoris as she repeatedly got into scrapes and he repeatedly had to rescue her. The fourth book completely changes everything and instead focusses on their son, Carthoris, and the woman he has fallen for: the titular Thuvia of Ptarth. This time it's Thuvia's opportunity to get kidnapped and Carthoris's opportunity to run around Mars to rescue her (and clear his name as the assumed kidnapper). Only the names have been changed to make it seem like a brand new book.
It does feel very derivative of the previous three novels. Thuvia is an unobtainable beauty, promised to somebody else. She is kidnapped by a jealous Jeddak and taken to a new area of Mars that we've never been to before. Our hero, Carthoris, is both blamed and also the only one actually capable of finding and rescuing her. The odds are as insane as ever as he goes up against two full clans of barsoomians and a whole new race. Oh yes, of course there's a new race. Every book has to introduce at least one new race of barsoomians to us. This time an even older race who believe they are the only surviving barsoomians. They have the power to create mental projections of their own kind and over time these are able to take on physical form.
Eventually, of course, no matter how insurmountable the odds they will be beaten; no matter how convincing the charges they will be proven false; and no matter how unobtainable the damsel, she will be unable to resist the charms of the son of John Carter. And no matter how contrived and repeatable the story, it does still have something of a "boy's own adventure" charm to it.