What is the main line of the book 'Little Dorrit' by Charles Dickens?

The web site The Literature Network summarizes Little Dorrit thus: Charles Dickens's masterpiece about prison life is set in an English debtors' prison (where Dickens's own father had been imprisoned) and where Amy Dorrit, the heroine, has spent her entire life caring for her imprisoned father. The novel portrays both the physical and psychological horrors of imprisonment and the hypocrisy of a society that allows them to continue.