Parshat Ki Sisa (Ki Tisa for Sephardim) is among the most involved and eventful Torah portions in all the Torah. NCSY's Ben Aaronson counts twenty events including the half-shekel census, the giving of the first tablets, the golden calf, G-d revealing himself to Moses both physically and with his thirteen attributes, and Moses's return with the second set of tablets. This eventfulness may be commonplace in Genesis and in the story of the Exodus, but Ki Sisa is neither. Furthermore, its surrounding four parshiot collectively tell little more than the construction of the Mishkan and its vessels. The preceding two explaining how the construction should proceed, and the forthcoming two recount the actual construction. As such, it is reasonable to ask why our sages grouped so much into only one parsha.
The answer is that this parsha is the single story of how and why the Torah was transformed to be for sinners about repentance rather than for angelic sinless people in an utopia. The Parsha starts with a command to count the people through a donation of a half-shekel. At this point, things are utopian; everyone can afford the same thing, everyone is equal, everybody's sins have been atoned for, and, after some purification, everyone is ready to receive the Torah. Moses ascends, and the written word of G-d is given to him.
Then the people, without Moses, decide they need a replacement and they build a golden calf. The people collectively and individually sinned, each by giving any amount of gold in a great fervor that could have lead to idolatry. G-d calls them corrupted and almost destroys the nation. The nation is no longer one fit for the original tablets. Moses has a lengthy plea to G-d ending with the request to see his face. The Talmud compares this with understanding how free will, divine providence, reward, and punishment can all be intertwined. G-d answers that Moses can see his back, which is interpreted to mean that at the end of days we will be able to look back and understand. Immediately after this G-d asks Moses to return to the mountain with new tablets whereupon G-d passed before Moses and proclaims his thirteen attributes of mercy. And by the hand of a man, the final two tablets were carved. The Talmud also says that on the mountain G-d put on a talis and showed Moses how to pray.
So, by the end of the parsha, the people know how to repent, know that G-d is generally forgiving, and know that even the greatest of achievements, such as inscribing the tablets, are supposed to be done by people. With this knowledge, the construction of the tabernacle could begin.