Well, ORION: Prelude is "multiplayer only". It launches with 3 game modes, two of which are objective / competitive based (VITAL & Lockdown CTF).
The other is a "substitute" for the lack of a single player element, and that is Cinematic Co-Op. It features exclusively designed objective-based missions intended for 4 players. This isn't a horde or a survival, there are actual objectives, waypoints, and moving from A to B type situations.
Regarding the story, many of the guys above me have it right. There is a HUGE story for ORION, with over 12,000 years of story in the universe.
Also, in that same breath, ORION is an already-designed Trilogy, with ORION: Prelude being directly linked to the development of ORION 1, which will feature the first part of the story.
Hope that helped clear a few things up