Theoretically, it's the same format as The League of Extraordinary Gentlemen, Volume III: Century. Three 72-page volumes, making one self-contained story... but this is also a standalone adventure and, frankly, more reader-friendly. Moore and O'Neill have been listening to their critics. Personally, I think the resulting trilogy is the best League instalment. (The original combo of Volumes 1-2 has a cooler cast, but for me the Nemo trilogy's story is emotionally richer and more consistent in its writing.)
This first book is set in 1925. Nemo's daughter, Janni Dakkar, has inherited his ship, crew and piratical ways, which has earned her the enmity of Ayesha from H. Rider Haggard's She. Soon, she's being hunted by three sons of bitches, the grown-up versions of the inventor heroes of boys' adventure series. Janni, though, has other obsessions. She wants to outdo her hated father in something and she's learned that he was once the only survivor of an Antarctic expedition. Aha, a chance to trump him! No, Janni, bad idea. This is actually a chance for Alan Moore to riff off H.P. Lovecraft's At the Mountains of Madness and Edgar Allan Poe's The Narrative of Arthur Gordon Pym of Nantucket. (If you also want to see this Shoggoth as a relative of The Thing, the text isn't stopping you.)
Moore doesn't do much with the horror content. It's presented straight and seriously, including bits you might have forgotten (e.g. Lovecraft's penguins), but the actual plot is mostly desperadoes hunting down pirates. That works, though. The hunters are utter bastards, while Janni's crew are willing to sacrifice themselves in the struggle against them. Janni does some maturing.
I liked this book. Even its bonus text section is unusually readable, since it's by a gender-swapped Hildy Johnson from The Front Page. (UPDATE: apparently, the gender is because this is specifically the version played by Rosalind Russell in His Girl Friday.) This first instalment is nothing special, to be honest, and the weakest of the Nemo trilogy, but it's the start of something good.