The Day of Σ (alternatively spelled 'The Day of Sigma') is a 25-minute long OVA movie included on the Maverick Hunter X UMD. It becomes available to the player once the game has been cleared once. The OVA, which serves as a prologue to the game itself, shows the events which lead directly to the beginning of the game and explains Sigma's reasons for rebelling against humanity as well as X's powers.