When I took this book out of the library I did not realise that it was the second part of a three-part series, and that one really needed to have read the first part in order to be able to follow of this one. Take this as a caution: this review is based on an incomplete knowledge of the story.
The story opens with Emily in Olympus, which is ruled by Jupiter, who won't allow her to go to New York to rescue her father from the CRU, "a secret government agency obsessed with capturing aliens" -- which sounds just like ICE, and probably is. She arranges with Pegasus, the winged horse, to fly her to New York to rescue her father. Pegasus is always referred to as "the stallion", lest anyone doubt his masculinity. In the end she is accompanied by three other friends, Joel, who appears to be a human from New York, Cupid, and Phaelen, who is not really described, but seems to be a bit like an octopus, able to squeeze into small spaces. and swans around with winged sandals, a gift of Mercury.
Emily is also the Flame of Olympus, which means she has a tendency to set herself and other things on fire. She also had a gammy leg, which is in a golden brace made by Vulcan. The injury was apparently caused by Nirads in New York, and the Nirads are apparently allergic to gold. The Nirads appear to be a threat to Olympus, and have sharp teeth and four arms.
I suppose a lot of this must have been explained in the first book of the series. The denizens of Olympus appear to be roughly based on the gods of Greek and Italian classical antiquity, with some, like Jupiter, Diana and Cupid having their Latin names, while others, like Apollo, have their Greek names. In the story, however, they appear to behave more like superheroes than deities.
In some ways the story is chillingly prophetic. It was first published in 2011, long before anyone imagined that Donald Trump would ever become president of the USA, with ICE as the fearsome government agency like the CRU in the book. But I could never really feel enough sympathy for the characters to really care what happened to them, partly because there always appeared to be some previously unmentioned superpower that could rescue them at the last minute before disaster, or even at the next minute, after the disaster had occurred.