Part I begins with Ivan's coronation as Tsar of all the Russias, amid grumbling from the boyars. Ivan makes a speech proclaiming his intent to unite and protect Russia against the foreign armies outside her borders and the enemies within - a reference to the boyars, who are already seen as discontented with his coronation. Shortly after, Ivan marries Anastasia Romanovna and there is a wedding celebration. This causes him to lose the friendship of his two best friends, Prince Andrei Kurbsky and Fyodor Kolychev. The latter receives Ivan's permission to retire to a monastery, while Kurbsky attempts to resume his romance with the Tsarina, who repels his advances.
The marriage feast is interrupted by news of the burning of several boyar palaces, carried into the Tsar's palace by a mob of the common people who also complain that the Tsar is being led astray by the Tsarina's family (the Romanovs), the Glinskys and the Zakharins. Ivan calms the crowd, but is interrupted by envoys from the khanate of Kazan, who send him a ceremonial knife with the suggestion that he do himself a favor by using it to commit suicide. Ivan immediately proclaims that his kingdom is at war with Kazan.
The next scene shows the siege of Kazan, in which Ivan's army digs saps underneath the city and fills them with gunpowder. Kurbsky, nominally in command, is reprimanded by Ivan for senseless brutality (he ties Tartar prisoners to palisades within earshot of the walls of Kazan and tells them to shout to the city to surrender; the defending archers immediately shoot the prisoners). The city of Kazan falls to the Russian army.
During his return from Kazan, Ivan falls seriously ill and is thought to be on his deathbed; Orthodox priests come to give him the last rites before he dies. Ivan sends for his relatives and orders them to swear allegiance to his son, the infant Dmitri, reminding them of the need for a single ruler to keep Russia united. They demur, with Ivan's aunt, Efrosinia Staritska, openly urging the others to swear allegiance to her son, Vladimir, instead. Emotionally overwrought, Ivan collapses and is thought dead. The relatives, celebrating, all begin to swear allegiance to Vladimir, the "boyar tsar" they have hoped for; meanwhile, Kurbsky is uncertain of his own loyalty, trying to decide between the two sides. However, when the Tsarina says, "Do not bury a man before he is dead," Kurbsky realizes that Ivan is still alive, and hurriedly swears his allegiance to Ivan's infant son, Dmitri. He is sent, as a reward, to the western border of the kingdom to defend against the Livonians and Poles. At the same time, Ivan dispatches Alexei Basmanov, a commoner he sympathizes with, to the south to take care of the Crimean border.
The Tsarina now falls ill, and while Ivan is receiving bad news from all fronts, the boyars plot to kill her. Efrosinia comes into the palace with a cup of wine hidden in her robes, in which she has put poison. Just as the royal couple receive word that Kurbsky has defected to the Livonians, Efrosinia slips the cup of wine into the room and listens from behind a wall. The Tsarina has a convulsion and Ivan, looking around for a drink to calm her, takes the poisoned wine and gives it to her.