The tutorial, "Overriding Existing Service in Drupal with Custom Module," by Saranya Ashokkumar in D4Drupal, offers a comprehensive guide on customizing and overriding services based on client requirements or custom logic. It outlines the process of overriding existing services in Drupal, providing insights into utilizing current user variables, printing messages, and creating modules. The tutorial delves into creating a new service provider, overriding existing services with a custom class, and implementing custom services with user details. Additionally, it emphasizes the best practices for overriding existing functions in a service and extending services with default variable values.