Terran Mission 7: The Trump Card
As far as the story goes, it turns out that the Zerg breeding program we found out about in mission 4 was actually the focus of the installation discs we got from that mission, and there weren't any "weapon" designs, per se, but there was a design for a Psionic Radiation Emitter or something which Mengsk somehow built behind the scenes. He then reveals that the Psi Emitter creates the radiations of Ghosts, which attract Zerg to their location ("So the Zerg are coming for you, darling? This just keeps getting better and better."), and thus the focus of this mission is to place a Psi Emitter in the middle of the enemy base location and use it to lure Zerg to the Confederate base.
As far as the gameplay goes, this mission is infamous for being able to be completely cheesed - because we start out with a science vessel, we can just cast the "Matrix" spell on the SCV carrying the Psi Emitter and voila, he's practically invincible now! Just make sure to hurry up! I get the feeling that this may be closer to what actually happens plot-wise, too.
Otherwise, this is where siege tanks and science vessels debut, and...well, I went overboard, researching EVERY tech relating to wraiths, siege tanks, and/or marines, and in the final assault I lost very few troops - battle tactics aren't really worth talking about! I'll admit that I sent in a sortie of six tanks and six wraiths beforehand to rampage through the enemy base, and while they all were destroyed, they did cause enemy attacks to come less frequently - a good indicator that the enemy was kinda gassed.
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Currently Playing - StarCraft
Hopefully Next Up - Final Fantasy VII, Resident Evil 2, Metal Gear Solid