You know the story of The Man in the Iron Mask, right? Decadent, tyrannical boy-king imprisons man with identical face behind an iron mask, that man is eventually broken out, and replaces tyrannical boy-king with no one the wiser, while the real king is imprisoned in his stead?
It's actually impossible for the great amount of lore accumulated around the legend of the man in the iron mask to have come from the book of the same name, because the subject of the title is dealt with in a few short chapters at the beginning of the novel, and then abandoned entirely. The plot to have him replace the king is completely UNsuccessful, and is mostly used to point out how rigid one of the Musketeers is in his old age.
Yep, one of those Musketeers. This is the fifth book in the series, and could more accurately be called The Four Musketeers Get Old and Die. Porthos, Athos, Aramis, and D'Artagnan all feature in this book and all - you guessed it - die in various manners. The major problem with this book is that it is heavy on plot and short on character interaction between the Musketeers, which is what made the series enjoyable in the first place.
In terms of long novels, this feels much longer than other ones of comparable length. Read it if you've enjoyed the Musketeer saga, but don't expect to enjoy it by itself, even with imagining Leonardo DiCaprio in the title role.