A normal tower defense game might not even have need for this section. A normal tower defense game would include a couple paragraphs about how hoards of X are assaulting your Y with unimaginable ferocity. They must be stopped, of course, or else horrible consequence Z will occur. But Immortal Defense goes the extra mile and introduces one of the most interesting and literary stories that I've seen in any game.
You are a Path Defender. The government of your planet, called Dukis, made you a Path Defender in order to fight an enemy known as the Bavakh. As a Path Defender, you exist in Path Space, a level of reality beyond all others. To you, the routes which starships travel to get from one point to another show up as nothing but simple paths, and as a result you're perfectly suited to defend your planet. This is done not with structures, but with manifestations of your psyche known as points. These points have their own personalities, and they express themselves in the game through simple text messages that appear on the screen. Upgrade a point and it might thank you. Place it well, and it will occasionally celebrate the number of kills it has been able to achieve.
This all sounds very detached from reality, but the game also makes it clear that although you are a Path Defender, you were once a mortal person. In order to defend your planet, you left behind your wife, your unborn child, your friends, and likely the chance to ever return to mortality. This is a point the game does not take lightly. The game's story is held together by paragraphs of text which appear at the beginning of each level, and these are often messages received from your daughter and, eventually, your grand-daughter. The story of Dr. Manhattan from The Watchmen seems to be a heavy influence on this game, but I dare say that the story of Immortal Defense goes even further in its exploration of the consequences of immortality.