The Day of Σ (Sigma) is a 25-minute long OVA included on the Mega Man Maverick Hunter X UMD animated by Xebec. 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 humanity as well as X's powers.