English subtitles
The death of the young Tsar Fyodor III has left Russia with a crisis of succession. Supported by Prince Ivan Khovansky, Fyodor's sickly brother Ivan, who is 16, and his half-brother Peter, who is only 10, have been installed as joint rulers, with their older sister Sofia acting as regent. Sofia has allied herself with Prince Vasily Golitsin, a powerful courtier and liberal politician, who is also her alleged lover.
Due to regulations applicable at the time of the composition of the opera in Imperial Russia, it was forbidden to portray members of the Romanov dynasty on stage, so Mussorgsky had recourse to a series of symbols and indirect mention of main characters in the plot. Sofia, Ivan and Peter never actually appear on stage.
The principal theme of Khovanshchina is stated outright in the choral number "Akh, ty Rodnaya, Matushka Rus'" in act 1 ("Woe to thee, native, Mother Russia"), which laments that Russia is bleeding and dying not because of a foreign enemy, but because of fragmentation within. Something like a three-way civil war is in progress, which basically compresses twelve years of Russian history into one telling. The Tsarist court is modernizing, and two powerful forces are resisting these changes: the Streltsy and the Old Believers. The Streltsy are decommissioned elite soldiers/guards ("Streltsy" literally means "shooters", just like "musketeers"), past their prime and on indefinite furlough. They are fanatically loyal to Prince Ivan Khovansky. The Old Believers are Russian Orthodox Christians who have left the state-sponsored church because they disagree with the Patriarch Nikon's reforms; they also challenge the line of succession to the throne and have refused to recognize the Russian Patriarch. Their leader is Dosifey. Fortunately for Czar Peter, these two factions despise each other, as the Streltsy are rowdy degenerates and the Old Believers are pious ascetics. Each of the three principal basses in the opera believes himself to represent the "true" Russia against her internal enemies: Prince Ivan Khovansky claims legitimacy by noble birth and military prowess, Dosifey by religion, and Shaklovity by supporting Czar Peter
The opera opens in Red Square with Shaklovity reading a letter to the court that wars of a rebellion planned by Prince Khovansky and the Old Believers. A scribe is told not to repeat what he heard and he signs the letter with a false name, fearing that he might be involved in political intrigue.
Meanwhile, Khovansky promises an upset crowd that he will defend the “young Tsars.”
Prince Andrey, who is the son of Khovansky, chases Emma with the intent of assaulting her. Marfa, his former fiancée, stops him and is then threatened by Andrey. Khovansky decided to capture Emma for himself. Dosifey, the leader of the Old Believers asks the Old Believers to join him in reuniting Russia.
Golitsin, a nobleman asks Marfa to tell him his fortune and she predicts that he will lose power and be exiled. He rejects her prediction and orders his servant to kill her. He is confronted by Khovansky, Dosifey stops them. Marfa is almost killed but she is saved. Shaklovity tells Khovansky that the Tsar is aware of the rebellion and is looking to arrest him.
In a later scene, Khovansky is murdered by Shaklovity and Golitsin is exiled as the conspirators fall. Marfa and Dosifey agree to immolate themselves at the close of the opera.
A full synopsis is given on Wikipedia.