From wiki: Act 1: The Feast: The oprichnik (bodyguard) Gryaznoi loves Marfa, daughter of the merchant Sobakin, even though Gryaznoi already has a mistress, Lyubasha, whom he has neglected of late. Marfa is already beloved of the boyar Lykov. In a jealous rage against Lykov, Gryaznoi arranges to cast a spell on Marfa with a magic potion from Bomelius, the Tsar’s physician. Lyubasha has overheard Gryaznoi’s request.
Act 2: The Love Philtre: Lyubasha in turn obtains from Bomelius another magic potion with which to cancel any feelings of Gryaznoi for Marfa. Bomelius consents, but at the price of an assignation with Lyubasha for himself.
Act 3: The Best Man: In the meantime, the Tsar of the title, Ivan IV (known as “Ivan the Terrible”), is looking for a new bride from the best aristocratic maidens in Russia. The Tsar settles upon Marfa. At the celebration of the engagement of Marfa to Lykov, everyone is surprised when the news arrives of the Tsar’s choice of Marfa as his bride. Gryaznoi had slipped what he thought was the love potion from Bomelius into Marfa’s drink at the feast.
Act 4: The Bride: At the Tsar’s palace, Marfa has become violently ill. Lykov has been executed, at the instigation of Gryaznoi, on charges of attempting to kill Marfa. When Marfa learns that Lykov is dead, she goes insane. Eventually, Gryaznoi admits that he had slipped a potion into her drink, and after learning that it was poisonous, asks that he himself be executed. Lyubasha then confesses that she had substituted her potion from Bomelius for Gryaznoi’s. In a rage, Gryaznoi murders Lyubasha, and is then taken to prison eventually to be executed. In her madness, Marfa mistakes Gryzanoi for Lykov, inviting him to return the next day to visit her, then dies.