You get the phone call at 4 a.m. By 5:30 you're in the secret installation. The commander explains tersely, "It's about the Slipgate device. Once we perfect these, we'll be able to use them to transport people and cargo from one place to another instantly.
"An enemy codenamed Quake, [sic] is using his own slipgates to insert death squads inside our bases to kill, steal, and kidnap.
"The hell of it is we have no idea where he's from. Our top scientists think Quake's not from Earth, but another dimension. They say Quake's preparing to unleash his real army, whatever that is.
"You're our best man. This is Operation Counterstrike and you're in charge. Find Quake, and stop him ... or it ... You have full authority to requisition anything you need. If the eggheads are right, all our lives are expendable."
Prelude to Destruction
While scouting the neighborhood, you hear shots back at the base. Damn, that Quake bastard works fast! He heard about Operation Counterstrike, and hit first. Racing back, you see the place is overrun. You are almost certainly the only survivor. Operation Counterstrike is over. Except for you.
You know that the heart of the installation holds a slipgate. Since Quake's killers came through, it is still set to his dimension. You can use it to get loose in his hometown. Maybe you can get to the asshole personally. You pump a round into your shotgun, and get moving.