14 DAY TRIAL // I've talked about how Blizzard was very adept at taking familiar elements from franchises the developers probably love and then integrate them seamlessly in Starcraft II: Wings of Liberty. Another good example is the zombie like missions in the single player campaign, which tasks the player with surviving at night while destroying a set of buildings during the day. The whole setup aims to explain how the Zerg evolved a more powerful virus to infect humans (which is redundant as a huge army is also overrunning the worlds where humans live and exterminating them) which is somehow affected by the light of the star so that the infested can only come out and attack at night.
The premise is convoluted but the mission itself is quite fun to play through. The night cycles are for defending, with bunkers, medics and SCVs ready to repair siege tanks and buildings the main units. The days are for quick raids on the Zerg/zombie settlements, taking down buildings to limit the amount of enemies that come out at night. Blizzard even throws a massive spin into the mechanics, introducing two huge Zeg units that only come out at night and need to be destroyed to get research points. It basically means that players need to venture out although the odds are clearly against them to hunt the Zerg down.
The only downside is that the mission gets way easier if the player has unlocked a significant number of units and upgrades earlier in the campaign. The hitpoint upgraded bunkers are quite helpful as are siege tanks. Meanwhile the Helion unit, a fast moving flame thrower that the mission introduces, is not too helpful and the hunt for Zerg infested buildings is quiker accomplished by using Reapers, a fast infantry unit that can handle small units of Zerg and can quickly destroy the buildings from where they spawn. In a big enough group they can also survive at night if the need arises.