I never played the original game so I had no idea of what I was going to see, not that I expected much from it being a 2 episode OVA but... to use an euphorism, it's rather confusing. The main character's (Burn) goal is to find his friend (Keith) who was kidnapped by some MIB; a friend who, from the OVA perspective, Burn knew for about a day but if that's enough for him to forge a friendship so strong good for him. Keith, on the other hand, has declared war to humanity to kill them all and create a world only for psychics; and toget revenge for being used as a guinea pig by the government, which is actually quite understandable. Wrong, but understandable.
Now, this could have been more than enough given the short format, but the authors felt the need to put in also all the other characters who I'm sure in the videogame all had their fair share of time to be properly introduced, including some who appear just once or twice and don't actually do anything relevant for the plot to proceed, so we're given only bits of information here and there which in the end lead to a big fat nothing.
To go into details I'd have to make spoilers so I'll stop here, but I strongly advice anyone who does not know the original story NOT to watch this else you want to get a headache from not understanding half of what's going on on screen.