The Straw Men is fundamentally a crime novel. Specifically it is of the sub-genre "serial killer", though this being Marshall it's far from the usual stereotype for such works. It's an intelligent, complex thriller where the plot becomes more twisted as the book continues. Marshall even manages to work in topics such as the evolution of socialisation, the architectural ideas of Frank Lloyd Wright and global conspiracy!
At the heart of the novel is "The Upright Man", a serial killer who has been responsible for the abduction and murder of a number of teenage girls. Once they are dead, he sends their parents a gruesome present: a sweater with the girl's name embroidered on it using their own hair.
There are three threads to the story. The primary viewpoint is a first person account by an ex CIA agent. When his parents are killed in a motoring accident he returns to the family home for their funeral. Whilst there he discovers a hidden note from his father saying "We're not really dead". He enlists the help of a friend and together they attempt to work out what is going on and hence begin to learn about The Straw Men.
The second plot thread involves an FBI agent and a retired policeman who are trying to solve the case of the Upright Man. After a period of inactivity he has struck again and a young girl is missing - hopefully not yet dead. The Upright Man's victims are usually killed about a week or two after their abduction, thus giving the story a nice time lock. The ex-policeman has personal reasons for wanting to catch the killer: his own daughter was murdered by the Upright Man.
The final thread is, of course, the killer himself. For obvious reasons not too much information is given about him, most of this thread is seen from the terrified viewpoint of his latest teenage victim. Marshall does an excellent job of getting the reader in to her head as she slowly loses all hope of rescue.