Short answers, pulled from the story.
Pope Innocent III issued a call for a new crusade in January 1198, marking the beginning of what would become the Fourth Crusade. The stated goal was to recapture Jerusalem by first defeating the powerful Egyptian Ayyubid Sultanate.
By May 1202, only about 12,000 crusaders had gathered at Venice, far fewer than the contracted 33,500. Many participants had sailed from other ports like Flanders, Marseille, and Genoa rather than Venice itself.
The city fell on the 24th of November 1202 after a brief siege despite citizens hanging banners marked with crosses from their windows and walls. Extensive pillaging followed, with Venetians and other crusaders coming to blows over division of spoils before leaders agreed to winter there.
In January 1203, Byzantine prince Alexios IV Angelos offered to pay the entire debt owed to Venice if the Crusaders would sail to Constantinople and restore his father Isaac II Angelos to the throne. The offer included 200,000 silver marks for the crusaders and 10,000 Byzantine professional troops.
The entire city was taken on the 13th of April 1204 following fighting that began on the 12th of April when weather favored the Crusaders. For three days following the capture, ancient Greco-Roman works were stolen or ruined, many civilians killed, and property looted totaling approximately 900,000 silver marks.