The Calendar Stone was an ancient Aztec artifact sought by the Nazis in Mexico during the 1930s. It was rumored that the Calendar possessed magic powers and could allow its user to manipulate history.
Indiana Jones foiled the Nazis' plans by infiltrating their dig site and fighting his way into the chamber where the Calendar Stone was discovered. He confronted General Anton Schlossberg and defeated his bodyguards. The General fled, warning Indy that they would meet again.
Jones claimed the stone and took it to Marcus Brody.