Watch Inside the Mind of Agatha Christie for Free: How to Stream the Movie Online at 123Movies. . Surpassed only by the Bible and Shakespeare, Agatha Christie is the most successful writer of all time. We all know her characters and incredible plot twists, but what do we know about Agatha herself? Combining rare access to Agatha's family, her personal archive and speaking to those who know her work best, discover what made the world's most successful crime writer tick.