(Post Doom Eternal) After retrieveing the ai, VEGA from Urdak, the demons attack the portal machine, sending the fortress of doom and it's occupants in a new dimension, where a giant purple robot is fighting against a blue floating octahedron in the middle of Tokyo 3. A bit OOC, especially for the Doom cast.
As the Fourth impact is ongoing, the Fortress of Doom and its occupants are catapulted right into the planet and without any information about their location, they have to resort to the only ones left in the area, initiating a series of events that will shape the ruined world forever. You don't need to know too much about either franchise, I explain things along the way.