What is the main conflict in Dirk Gently's Holistic Detective Agency by Douglas Adams?

In Dirk Gently's Holistic Detective Agency, private investigator Dirk Gently draws his old college friend Richard MacDuff into a twisting investigation to ultimately save the world. A ghost, millions of years old, has been roaming the Earth in guilt and loneliness. Driven mad, he wants only to go back in time and correct the mistake that killed him and his friends, a group of aliens on their way to establish a Utopian paradise on another planet.