The Summa Theologica is meant to summarize the history of the cosmos and provide an outline for the meaning of life itself.
This order is cyclical. It begins with God and his existence in Question 2. The entire first part of the Summa deals with God and his creation, which reaches its zenith in man. The First Part, therefore, ends with the treatise on man.
The second part of the Summa deals with man's purpose (the meaning of life), which is happiness. The ethics detailed in this part are a summary of the ethics (Aristotelian in nature) that man must follow to reach his intended destiny.
Since no man on his own can truly live the perfect ethical life (and therefore reach God), it was necessary that a perfect man bridge the gap between God and man. Thus God became man. The third part of the Summa, therefore, deals with the life of Christ.
In order to follow the way prescribed by this perfect man, in order to live with God's grace (which is necessary for man's salvation), the Sacraments have been provided; the final part of the Summa considers the Sacraments.
The entire Summa can be summarized roughly in this chart: