Madison Kelly, a San Diego private investigator, arrives home to a note stabbed to her front door: Stop investigating me, or I will hunt you down and kill you. The only problem? Madison hasn't been investigating anyone - she's been taking time off to figure out what to do with her life. But how does she prove a negative? The only way to remove the threat is to do exactly what Anonymous, the note writer, is telling her not to do: investigate to see who left it. Could this have something to do with the true-crime podcast she's been tweeting about and the missing girls?

The girls went missing two years apart, after a night at the clubs in San Diego's famed Gaslamp Quarter, and Madison had been probing the internet for clues. She discovers that someone has been one step ahead of her, monitoring her tweets to prevent her from getting too close. Soon Madison's investigation brings up more questions than answers: are the disappearances connected? Are the girls dead or did they just walk away from their lives? And who is Anonymous, the person who will stop at nothing to keep Madison from learning the truth?

As she closes in, so does Anonymous. Set against a backdrop of surfer culture and coffee houses in San Diego, Anonymous follows Madison as she confronts the reality of the girls' disappearance in a terrifying climax where the hunter becomes the hunted - and Madison is running for her life.