Modo and Octavia are sent to investigate ships being sunk by "sea monsters" in the waters off Iceland. This being a steampunk story, it's obviously not a living creature but an underwater submarine (as submarines generally are :-)
Modo gets split up from Octavia who doesn't play such a big part in this book. Instead the heroine is Collete, a French spy who is a bit more mature - well she is 18. It's very Jules Verne.
I think I slightly preferred the first book in that it was a magical new story as I was introduced to the wonderful characters. But that's not to say that this is any less a good book. I shall definitely get book 3 and I hope there's much more to come.