The first step is to invoke the incident response plan to ensure a systematic, controlled, and compliant response to the security incident. "The incident response plan should be activated immediately to investigate, contain, and resolve incidents of unauthorized access." - CISM Review Manual 15th Edition, Chapter 4: Incident Management, Section: Incident Response Plan Execution* ISACA practice questions also reinforce that invoking the incident response plan is the essential first response to contain the breach.