'Novus Actus Intervenien' (NAI) essentially means an intervening act. This applies after you have gone through the stages required for Factual causation and Legal causation. You will know that a NAI applies when there is either a third party involvement, the victim own act interferes or when there was a natural but somewhat unpredictable event that has occured. For a NAI to be successful, it must show that the defendant is not responsible for the consequence (which in most cases will be some form of injury).