After his father is brutally murdered a young man is dragged into a conflict between ancient enemies. Saul Garamond is a young Londoner in his mid-twenties. He comes home one night from visiting friends out of town and sneaks into his room to avoid having to talk to his father.
In the morning he is shocked awake by the police banging on the door. He sees the shattered glass of his flat's window and realizes something is terribly wrong. He wonders if his father has committed suicide but soon is told that he has been murdered, and Saul is the main suspect. He is questioned by a detective named Crowley, and things look very bad for him. He has no alibi and although his relationship with his father was very good when he was younger, in the past few years they often argued over his father's leftist politics and grew apart.
But then a strange man appears in Saul's cell. He smells terrible and his eyes are shadowed, but he reveals himself to be King Rat and tells Saul that he is his uncle, and that Saul's mother was his sister and also a rat. She died when Saul was born and with his father now gone he has no other family. Knowing how bad things look for him and in a state of shock from the day's events, he goes along with the strange man.
King Rat carries Saul over the rooftops and shows him the hidden world of the rats. He reveals Saul's own rat abilities (eating anything and fitting into small spaces and shadows to the point of invisibility) and also tells Saul that the murder was committed by an old enemy in order to set Saul up.
Back in the regular world Saul's friends Natasha, Fabian, and Kay are worried about his disappearance and uncertain what to think of it. They are also being questioned by Crowley who becomes less certain that Saul is the killer the more he investigates.
Natasha is a DJ and is working on a drum 'n' bass record searching for a new sound. Out of nowhere a new collaborator shows up, a flute player named Pete, whom Fabian and Kay don't like. Natasha finds him a bit creepy as well, but despite being a novice to electronic music Pete is enthusiastic about the ability to layer tracks and the music they're making seems to be something special.
As Saul settles into his new life learning to forget his human traits and build up his rat ones slowly becoming more and more like King Rat, but he still misses his friends and old life.
When rumors arise about the return of someone called the Ratcatcher and King Rat takes Saul to speak to two other animal monarchs; Anansi the Spider King, and Loplop the King of the Birds, both of whom also fear this old enemy. During this encounter King Rat tells the true story of the Pied Piper of Hamelin, and how he was the only rat survivor when all of his subjects were drowned. Because of it he lost their loyalty, they no longer obey him.
Saul still misses his old life and makes the mistake of visiting Kay but the encounter just proves that Saul has changed too much to return to a normal human life. The Piper murders Kay after that visit, and later kills another of Saul's friends, a homeless girl named Deborah whom he just met.
Saul goes to his father's apartment and while there finds and reads an old diary in which he discovers that his mother was raped nine months before Saul's birth. The man who raised him was not his biological father after all. He is shattered all over again by his father's death, knowing now that the man never even hinted he was not his son. Worse, he realizes that his mother wasn't a rat after all, King Rat is Saul's father and has been lying to him about everything and Saul's entire existence was orchestrated in order to create a weapon to use against he Piper.
The Piper finds and attacks Saul, and we learn that Saul is special because of his half-breed status, he is immune to the Piper's music. But working with Natasha in the guise of Pete, the Piper has been developing a hypnotic new music that will work on humans as well.
Saul escapes from the Piper with the help of Loplop and goes to confront King Rat with the truth he now knows. He is disgusted with everything he now knows and after they fight, Saul is ready to leave the whole mess of them to their battles. He goes off on his own to brood for a few days but soon notices a change in the sewers, he traces it back to the throne room where he finds the Piper has set up a cassette player to keep King Rat, Loplop, and the rats all hypnotized.
He releases them and rushes off to the club along with the animal monarchs and an army of rats and spiders. He knows Natasha is going to be spinning at the Jungle rave and the Piper will be trying his new music for controlling humans.
The music works its magic and every creature in the club is under the Piper's spell including Saul. But the Piper wants more than that and he plays a second layer of his flute music to force Saul to dance and to kill him. But the Piper doesn't quite know how to mix smoothly, and the new flute music on top of the old snaps Saul out of it enough to escape being killed. The Piper chooses to escape rather than fight. He opens a portal in the wall, the same as he did long ago to lead the children of Hamelin away, and horrifyingly Saul hears the wailing of them still trapped there. He hesitates in horror and King Rat steps in, stabbing the Piper with his own flute.
When they turn off the music the animals flee, leaving the humans to recover. King Rat and Saul sneak off as well. King Rat demands that Saul tell the rats what he did, that he got rid of the Piper, knowing they will trust and believe Saul but not him.
King Rat wants his kingdom back but Saul hates King Rat, knows he will be a bad ruler, and believes he won't kill Saul as long as there is any doubt that the Piper is truly dead. He refuses and instead declares he will tell the rats that King Rat cowered and begged for his life. Saul does not take the crown either though, remembering his adopted father's Marxist politics he declares himself instead simply Citizen Rat.
Finally, Saul calls Crowley and tells him he did not kill anybody that the killer is gone for good, and for all intents and purposes so is Saul. He tells the detective to stop bothering his friends.
Best part of story, including ending:
It's a solid coming of age story with a bit of a twist in that the long lost heir to the throne renounces it in the end.
Best scene in story:
I love when King Rat first whisks Saul over the rooftops, it's heady and exciting and a little bit sinister. You definitely see that you're crossing into a different world.
Opinion about the main character:
Saul is a little bit immature, more like a teenager than a twenty something, but he is half-rat which might account for some of that. He definitely matures over the story and learns to embrace his strengths and use them for good.