Although the battle of Kalka, which became the first clash of Russia with the Mongol-Tatars, is included in the compulsory program of all school and university textbooks, it remains largely mysterious for historians. Disputes are about the place of the battle, since there is no river with this name on modern maps. Discussions unfold not only about the day, but even about the year of the battle. There is no unity of opinion about the number of fighters, the route of movement of opponents. How are the famous comet Halley and Karl Marx connected with 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 totality of historical sources, the author answers these and other questions.