Complex sentence: Since Clean Willy is currently incarcerated in Newgate Prison, Pierce, using an old alias called "John Simms", sends a message through Willy's former mistress and assists him in escaping from Newgate while the public is distracted by an execution.
Split into simpler sentences: Since Clean Willy is currently incarcerated at Newgate Prison, Pierce uses the alias "John Simms" to send a message through Willy's former mistress. Pierce then assists him in escaping from Newgate while the public is distracted by an execution.