I just finished MDK 1 and had to read wikipedia to even know what all this was about and who characters were. I mean there are no cutscenes or dialogues in whole game only these briefings (that don't tell much anyway) between missions. It even ends out of blue without any credits or even "congrats" screen and summary like previous missions. I seriously had to double check if this was really ending or something was bugged.

Actually after that I downloaded manual and found some story and background there, so I guess this is answer for my question. But still it somehow makes this, overall very fun, game feel incomplete.

It's like you said - background story is in the manual, and it does add quite a bit of flavour.

Love the ending by the way - that cinematic (mix of CGI, live action, and then music video with BZK) is just unique, cool, weird, bombastic, unexpected - reflects the spirit of the game pretty well :)