Please, help me with these phrases: "haunted man" and "seedy underbelly"

Here is the whole passage:
"Soon after exchanging wedding vows with his second wife, a haunted man becomes obsessed with discovering the true fate of the missing daughter from his first marriage. Later, when the man and his new wife consult with a mystic, it quickly becomes apparent that some things are better left forgotten. With the help of a sheriff and two FBI agents, Jack searches the seedy underbelly of Mexico City for his daughter and discovers that there's no escape from his past."