This is an automatically translated description

Sophie Foster returns to the Lost Cities and is surprised to find that everything has changed. The world is threatened by war, and the Unseen continue to wreak havoc. The girl is completely confused, finding it increasingly difficult to distinguish between friend and foe, between those who can be trusted and those who are playing a double game. But when Sophie is warned that Grady and Edeline are the new targets of the Unseen, she realises that she must act now.

If only she could decipher the secret symbol of the Unseen! And although each new clue only makes things more confusing, Sophie eventually learns that it's not just the Black Swan that has devious plans... The Unseen have launched the Arctos initiative - presumably to gain control of the Moonlark. So the girl and her friends must stop the group that threatens not only her but the whole world at all costs.