The action of I Puritani takes place in the middle of theseventeenth century, towards the end of the English Civil War. England isdivided between the Royalists, faithful to the Stuart monarchy and leaningtowards Catholicism, and the Roundheads, the supporters of Oliver Cromwell andsiding with Puritan religious views. King Charles I has been executed and hiswife Queen Henrietta has escaped in disguise. The setting is Plymouth, a Puritanstronghold.
 The Introduction opens with an orchestral Sinfonia,setting the scene in a fortress in Plymouth. Battlements and turrets are seen,against the mountains in the background. The sun is rising.  The voice of thePuritan officer Sir Bruno Roberton is heard, with the soldiers on guard, as thetrumpet signals mark the dawn of a new day and the men express theirdetermination to defeat the Stuarts.  A bell rings and the sound of theorgan is heard, as the Puritans are heard in prayer, with Elvira, daughter ofthe governor of the town, Lord Gualtiero Valton, the retired Puritan colonelSir Giorgio, his brother, and the Puritan colonel Sir Riccardo Forth.  Thepeople in the fortress sing in celebration of the coming wedding of Elvira.They go, leaving Bruno, who stands aside, observing Riccardo.  Riccardolaments Elvira's failure to return his love. Bruno interrupts his musing andsuggests that he still has his country and heaven as consolation. Riccardo explainshow Elvira's father had agreed to her marriage to him, but when he hadapproached him, Lord Valton had told him that he had no command over hisdaughter's heart, and that she was in love with the cavalier, Arturo Talbo. Riccardo laments again his loss of Elvira.  Soldiers march past and Brunourges Riccardo to think of his country and honour, but the latter continues todwell on the memory of his tender love.
 The new scene is set in Elvira's room. The Gothic windowsare open and the battlements of the fortress can be seen. Elvira, in somedistress, addresses Giorgio, her uncle, telling him she will never marryRiccardo.
 She would rather die than marry him, but he tells herthat he has persuaded her father to allow her to marry Arturo, in spite of hissupport for the royalists. Elvira is overjoyed, and Giorgio explains how he hadconvinced her father, telling him that Elvira would die if she were forced intothis marriage.  The sound of hunting horns are heard, heralding the arrivalof Arturo, granted a safe conduct to the Puritan fortress. Elvira and Giorgioexpress their joy at the occasion.  All gather for the wedding, as Arturoand his retinue, squires and pages, appear, and there is a chorus of praise forthe couple, beauty now to be united to valour.  Arturo addresses Elvira andthe onlookers express their delight at the happiness of the couple.  Valtongives Arturo the necessary pass and tells his brother Giorgio to accompany thecouple to their wedding. Turning to Enrichetta, he courteously tells her thatshe is to appear before Parliament, which she realises will mean her death.Arturo privately asks Giorgio about the prisoner.  As the latter leaves,together with Elvira and her attendants, and Valton with his guards goes toprepare for Enrichetta's arraignment, she addresses Arturo, who has lingeredbehind, seeking his help and revealing her identity as the captive Queen. Hepromises to help her.  Elvira enters, dressed for her wedding, wearing agarland of roses and carrying the fine bridal veil that Arturo has given her.She seeks Enrichetta's help in wearing it, before leaving with Giorgio and herattendants.  Once they have gone, Arturo realises the possible use of the veilas a means of disguise and escape for the Queen.  At this moment Riccardoappears, with drawn sword, determined to seize the supposed Elvira. The two menare about to fight, when the Queen draws aside her veil, showing Riccardo thatshe is not Elvira. He recognises the prisoner, but allows them to pass. Elvira returns, with her uncle and father, and others, to find Arturo and theprisoner gone, bringing hue and cry after them.  Elvira is now out of hermind, and raves in her madness, calling on Arturo, to the pity of theonlookers.  She seems to see him running from her, while the others lamenther fate and vow revenge.
 In another part of the fortress people express theirpity for Elvira.  Giorgio appears, and they ask what news he has.  Hedescribes the scene of Elvira's madness, again exciting their pity, and angerat Arturo's treachery.  Riccardo enters, with a document condemning Arturo,vowing revenge on him.  Elvira, meanwhile, is heard continuing her lament,calling only for death, overheard by Riccardo and Giorgio.  She enters,recalling Arturo's promises, and not recognising the two men. She imagines sheis with Arturo,  calling on him to hurry to her.  Riccardo vows revenge,while Giorgio tells him that the death of Arturo will bring about the death ofElvira.  He will be haunted by Elvira's ghost.  Giorgio is convinced byRiccardo's reply, and the two agree to join together in vengeance.  Let thetrumpets sound, calling them to arms to fight with Arturo and avenge Elvira.
 In a garden near the fortress a storm is breaking.Arturo rushes in, casting aside his cloak. Now he is safe, and declares againhis loyalty to his country and his love.  He hears the voice of Elvira,singing his own song of a lovelorn troubadour. He calls out to her, and whenthere is no answer, he sings the same song himself.  He hears peopleapproaching, dons his cloak and hides. His pursuers declare their intentions,but do not see Arturo.  Once they are gone, he emerges from hiding, but isin doubt what to do. He sings again his song.  Now Elvira hears him. To heramazement she sees Arturo, who kneels before her.  She asks how long theyhave been apart, three months that have seemed to her three centuries, in whichshe called out for him. He seeks her forgiveness.  He would embrace her,and she can find no words to express her joy. Drums are heard, and Elvira'sdemeanour changes, to Arturo's increasing alarm. He understands that hisenemies are approaching, while she welcomes the sound as for their wedding. His pursuers draw near, and she clasps his knees, calling on them forhelp, in case Arturo escapes from her again. Arturo is surrounded, and Elvira'smood changes once more, when she hears his death pronounced.  Arturo ismoved by Elvira's plight. Under the threats of the Puritans, he tells them todesist, and have some pity for Elvira.  At this moment the sound of heraldsis heard. A message is given to Giorgio and Riccardo, who announce, with joy,the defeat of the Stuarts and the freedom of the country, with the pardoning ofall prisoners, an event that brings Elvira to her senses once more, so that thelovers can be together.