At the end of Dreamless, Helen freed the Olympus gods from their prison and now must find a way to contain them again, or there will be a terrible war between the Scions and the gods. The gods have already started roaming the Earth, wreaking havoc on the unsuspecting population.
Moreover, the Oracle reveals that a Tyrant is rising: a being who will have powers from all houses. Members of the different houses watch each other with suspicion. Orion, especially, is believed to be the Tyrant the prophecy talks about, even though Helen is sure he isn't.
Helen is still torn between her feelings for Lucas and those she has for Orion, but she needs to make a choice.
Helen's powers grow a lot throughout the whole series, but especially in this book.Now she's developing the same powers as Lucas and Orion, while also retaining her original ones. On her head there's also Aphrodites' curse - there will always need to be a daughter of Helen in the world. So Helen needs to have a daughter, sooner or later, and it can't be with Lucas.
Some other exciting stuff happens, but I can't mention it or I'll spoil the book for you.