StarCraft: Brood War/Protoss Mission 6: Return to Char

Walkthrough
You start with two bases in this mission; a Protoss base and a Zerg base. They are very near each other, so it is easy to support each base when you need to. Once you start the game, you will notice that your Protoss base has many mineral fields, but does not have a geyser. Your Zerg base does have a geyser, but it only has a five-mineral field patch. You need miners from both bases to establish your economy.

Now take a look at your starting Observers; the one on the west side shows you two burrowed enemy Lurkers, and the one on the east side shows you a small mineral patch with a geyser. You have another Observer near your Forge. Lurkers are large-sized ground units evolved from Hydralisks, and they will attack you while burrowed. The Lurkers will probably make you jealous because you can't make them yourself... yet.

You should expand at your small base quickly for more gas. It is ideal to build a Zerg base there, since the area is both closer to your Zerg base and you can get Drones more quickly than Probes. You can support this base with Photon Cannons, but build them up the cliff.

Your next expansion should be to the west, where it is occupied by Orange Zerg. A group or two of hydralisks or dragoons can take it out without too much trouble, but bring an overlord or observer to detect the numerous lurkers located there. Quickly build a hatchery; don't waste your time waiting for the creep to recede. Here, you'll find a bountiful vespene geyser and plenty of minerals if you attack quickly enough.

With the boon of resources coming in from the new expo, build 6 carriers. While you're making your carriers, make sure your air and armor upgrades are at level 3 and also research Carrier Capacity for extra carrier interceptors.

You will notice that there is a path leading south to the Overmind from your new expo. Once you have your carriers, move them to the new expo and then move them down the cliff to the left of the path. This is because there are spore colonies and groups of scourge which cover the middle and right side of the path. Provided that you stick to the cliff and you don't attack anything, you will not draw any unnecessary fire. Once you get to the bottom of the cliff, the only thing that is standing between you and the Overmind is one spore colony and only a handful of hydralisks. Just ignore them, have all of your carriers focus fire on the Overmind, and it will go down in a matter of seconds.

Alternate Strategy
If you want to try to push to the Khalis, you should follow this strategy. Expand as fast as possible to the second geyser below the zerg base. But you can almost ignore the whole protoss base and everything west of it. Just put enough units/cannons on the west entrance to fight off the occasional few hydralisks/zerglings coming that way.

Now build 3 hatcheries and keep producing hydralisks; don't forget to upgrade them. Two or three groups of 12 hydralisks each should be enough to push at the brown zerg to the south. Send all of the hydralisks produced in your base to support the attack. Then you should be able to clean the winding path to the Khalis crystal with relative ease. Then just send a drone or probe there. This strategy is probably faster than going for the Overmind.