Although the Battle of Kalka, which became the first clash between Rus' and the Mongol-Tatars, is included in the mandatory curriculum of all school and university textbooks, it remains largely mysterious for historians. There is debate about the location of the battle, since there is no river with that name on modern maps. Discussions unfold not only about the day, but even about the year of the battle. There is no consensus on the number of fighters or the route of movement of the opponents. How are the famous Halley’s Comet and Karl Marx connected to the battle? Did the Russian hero Alyosha Popovich participate in the battle? What role did the Kalka River play in the fate of the Horde temnik Mamai? Is it possible now, after 800 years, to restore the events of the past? Based on an analysis of the entire set of historical sources, the author answers these and other questions. Reviewers: Ph.D. Dityatkin D.G., Shpolyansky D.V. Maps for the book were made by Doctor of Historical Sciences. S.N. Temushev, Professor of the Department of Russian History, Faculty of History, Belarusian State University