S.D.I. takes place in the year 2017 of an alternate timeline, in which the Soviet Union continued to exist and pose a threat to the countries of the free world. USA launched another Strategic Defense Initiative, its commander-in-chief being General Sloan McCormick. A Soviet extremist group consisting of KGB members has gained control of space launch facilities, and began firing nuclear missiles at America. McCormick must fulfill his duty, but his heart belongs to a woman who is on the opposing side. He controls a space-based fighter, trying to neutralize enemy missiles. Defense satellites must be repaired as well, and enemy bases infiltrated in order to rescue his beloved one and save his nation and the world.