My suggestion:
Companion cube has been kidnapped. You are trying to save it.

Good ol' gunfight - Filmed in CS:S / CoD4?

Car Chase - Filmed in BF2

Murder of Companion cube - Filmed in Portal when the companion cube gets incinerated.

Person trying to save Companion cube goes loco and blows the world up with a nuke - Filmed in Garry's Mod using a nuke mod.


Just a quick idea... It can be worked on quite a bit XD
They're all just suggestions, so it can be changed