What is the main conflict in The Children of Men by P. D. James?

The Children of Men is a dystopian novel, set in a less-than-ideal world of mass infertility. In this setting the reader follows Dr Theodore Faron, a doctor of philosophy and a historian of the Victorian age at Oxford University. Theo leads a solitary life. Even when he was married he was not close to his wife or his daughter. When he joins a group of vigilantes who are standing up to his cousin, the Warden of England, Theo learns how to be passionate, for good or for bad.