The Mystery Train plot
Joan Lane, wrongly convicted of theft, is protected by social worker Marian Radcliffe and her lawyer William Mortimer when the train taking her to prison is hit and damaged. Joan's new friends are passengers on the train who are actually planning to steal a famous diamond from William's client Ronald Stanhope. They offer Joan their help, thinking that she will help them with the theft. Joan and Ronald fall in love. However, Joan refuses his marriage proposal, fearing that Ronald, who knows nothing about her past, will break up with her if he discovers the truth about her.