This is Max's story (who we met in book one). Max happens to have a fascination with all things Atlantis. Sabine happens to be descended from Altanteans. Sabine and her aunts (one of whom is the 3rd Guardian) have moved to London because of the "prophecy." Someone is killing generals who can't be bought and Guardians to steal the elixir in an attempt to take England's army (and thus the world).
While I enjoyed this, I wish there hadn't been so much "I can't love you because of Reasons." There was too much of that. I did like that there were 2 "evil-doers" in this one (with completely different goals). All-in-all, this was a fun, Indiana Jones type romance with some supernatural and legend through in.