This is the fourth book in the Pegasus series, which follows the adventures of Emily, a mortal girl from New York who one day discovers wounded Pegasus on her rooftop. Via various adventures, Emily travels from Earth to Olympus and discovers that she is not mortal, but a divinity known as the flame of Olympus. With the help of her mortal and Olympian friends, Emily tries to maintain peace in both worlds.

In the last book, while threatened by government secret agents named CRU (Central Research Unit), Emily used her power to send Pegasus and Alexis the Sphinx to a mysterious jungle world. Since then she is been having strange dreams of this world, and how she must get to a gathering but continues to miss it. The mysterious world she dreamed about is called Xanadu, at the beginning of the solar stream. This world is considered to be more ancient than Olympus and it was guarded by the powerful race, the Xan, yet one day they disappeared. Jupiter is convinced that Emily is in fact, a Xan. 

A new character is introduced, in a parallel story. Stella Giannakou is a 16 year old Greek girl; her parents are archaeologists and take her with them all over Greece; yet she dislikes history and they do not believe in the old myths while she does. They discover an ancient chest at an old temple of Poseidon and try to open it; yet there is only big rock inside of it. 

Meanwhile Emily and the Olympians travel to the mysterious Xanadu. Emily remembers that she is Riza, the last of the Xan, who remained after they all left. She tried to follow them but instead her fragmented remains crashed on Olympus a long time ago, during the war with the Titans. Riza’s spirit became fragmented and part of her scattered to other worlds as well and on Olympus Vesta put her heart in a human child so she lived on in many incarnations as a human girl until she became Emily. 

At the same time, a mysterious illness is taking hold of the gods on Olympus. It happens that the rock found in the box by the archaeologist, was in fact a secret weapon concocted by the Titans in their ancient war against the Olympians. This rock could make the Olympians age and die. Since the box in which it was hidden was opened, the Olympians suffer its effect. Emily and Joel then travel to Greece in order to stop the weapon from killing more Olympians. 

When they arrive, they encounter Stella who tells them that the rock is at the Acropolis Museum in Athens. Emily and Joel head to the Acropolis with Stella. They discover that on the golden box uncarved an image of them alongside the gods as well. A CRU agent (named agent B) catches them at the museum and tells them that the deadly rock was sent to Olympus. So Emily, Joel and Stella return to Olympus in an attempt to save it from the dangerous weapon while CRU agents are swarming Olympus. Emily then learns that she has the power to go back in time, so decides to go back to ancient Greece and stop the Titans form ever creating the cursed tock. 

Emily, Joel, Pegasus, Paelen, Stella and Agent B, go back to the past, where they land amidst the Titanomachy. They meet the Olympians as their young selves, especially Jupiter and Neptune. The group then helps the Olympians of the past to beat the Titians amid a formidable fight. In the end the Olympians win, Emily destroys the weapon and the effect of the time travel ends when all the mortals involved (except the Olympians) cannot remember what happened. Life goes back to its normal route.

In every book in the series thus far Emily went through some kind of change; first she discovered she was the flame of Olympus, then she gained powers and learned she is not human. Now she discovers she is in fact part of an ancient race. This is a coming-of-age story, and as such, Emily is facing all kinds of challenges and obstacles she must overcome, while her aim remains to find out who she really is. Search for self-identity is a crucial role of growing up and becoming an adult and with Emily’s fantastic adventure this process is highlighted, as her identity is indeed shrouded in mystery; she confesses, “I don’t know who or what I am any more.” (Kindle Locations 310–311). Emily fears that things are changing, her friends do not treat her as they did, and all she yearns for is to belong; “but if I’m really Xan, what am I part of? Who am I?" (Locations 335–336). Agent T, who became a tree on Olympus in the last book, reassures Emily, “You are what you believe you are, Emily.” (Location 354). The most important message is that your true self remains the same even if other things change. You need to remain true to yourself and trust your family and friends to love you for who you are inside.

Finding who you are can be scary. Emily’s fear is more realistic due to her mysterious adventure, as she confesses, “It’s like I’m not myself any more. Like there are two people in me. I’m afraid I am going to disappear." (Locations 665–666). Emily will need to learn to accept all the different sides of herself in order to become whole again. The fear of disappearing could allude also to psychological change children go through when growing up; they fear their young, childish self will disappear forever and that they will change without recognition and become someone else. However, growing up does not have to be this way; you lose part of yourself and gain new one and thus keep evolving and growing. You do not lose yourself, you just grow up. Emily learns she must control her temper if she wishes to vanquish Saturn and his Titans. Growing up is a time of heightened emotions, and the author makes it graphically noticeable via Emily’s powers which magnify when she is upset. Learning to control her temper and find calm will help her and save the world.

The story also deals with the importance of myths. The Olympians, who became myth for the people on earth, have their own myth in Xanadu. Stella and her parents dig up the past to uncover the old myths. Hence, myths and the continuation of myth symbolizes the continuation of cultures, human or otherwise and are a significant part of the development of life. As Agent B states when the group goes back in time regarding the ancient people they meet "These people knew the Olympians were real. It’s us in the modern age who stopped believing." (Location 2299). 

The review refers to the ebook edition