Enter Angelotti. He has escaped from prison and is seeking a hiding place. Looking about, he recognizes a pillar shrine containing an image of the Virgin, and surmounting a receptacle for holy water. Beneath the feet of the image he searches for and discovers a key, unlocks the Attavanti chapel and disappears within it. The Sacristan comes in. He has a bunch of brushes that he has been cleaning, and evidently is surprised not to find Cavaradossi at his easel. He looks into the basket, finds the luncheon in it untouched, and now is sure he was mistaken in thinking he had seen the painter enter.
The Angelus is rung. The Sacristan kneels. Cavaradossi enters. He uncovers the painting—a Mary Magdalen with large blue eyes and masses of golden hair. The Sacristan recognizes in it the portrait of a lady who lately has come frequently to the church to worship. The good man is scandalized at what he considers a sacrilege. Cavaradossi, however, has other things to think of. He compares the face in the portrait with the features of the woman he loves, the dark-eyed Floria Tosca, famous as a singer. "Recondita armonia di bellezze diverse" (Strange harmony of contrasts deliciously blending), he sings.
Meanwhile the Sacristan, engaged in cleaning the brushes in a jug of water, continues to growl over the sacrilege of putting frivolous women into religious paintings. Finally, his task with the brushes over, he points to the basket and asks, "Are you fasting?" "Nothing for me," says the painter. The Sacristan casts a greedy look at the basket, as he thinks of the benefit he will derive from the artist's abstemiousness. The painter goes on with his work. The Sacristan leaves.
Angelotti, believing no one to be in the church, comes out of his hiding place. He and Cavaradossi recognize each other. Angelotti has just escaped from the prison in the castle of Sant'Angelo. The painter at once offers to help him. Just then, however, Tosca's voice is heard outside. The painter presses the basket with wine and viands upon the exhausted fugitive, and urges him back into the chapel, while from without Tosca calls more insistently, "Mario!"
Feigning calm, for the meeting with Angelotti, who had been concerned in the abortive uprising to make Rome a republic, has excited him, Cavaradossi admits Tosca. Jealously she insists that he was whispering with someone, and that she heard footsteps and the swish of skirts. Her lover reassures her, tries to embrace her. Gently she reproves him. She cannot let him kiss her before the Madonna until she has prayed to her image and made an offering. She adorns the Virgin's figure with flowers she has brought with her, kneels in prayer, crosses herself and rises. She tells Cavaradossi to await her at the stage door that night, and they will steal away together to his villa. He is still distrait. When he replies, absent-mindedly, he surely will be there, her comment is, "Thou say'st it badly." Then, beginning the love duet, "Non la sospiri la nostra casetta" (Dost thou not long for our dovecote secluded), she conjures up for him a vision of that "sweet, sweet nest in which we love-birds hide."
For the moment Cavaradossi forgets Angelotti; then, however, urges Tosca to leave him, so that he may continue with his work. She is vexed and, when she recognizes in the picture of Mary Magdalen the fair features of the Marchioness Attavanti, she becomes jealous to the point of rage. But her lover soon soothes her. The episode is charming. In fact the libretto, following the Sardou play, unfolds, scene by scene, an always effective drama.
Tosca having departed, Cavaradossi lets Angelotti out of the chapel. He is a brother of the Attavanti, of whom Tosca is so needlessly jealous, and who has concealed a suit of woman's clothing for him under the altar. They mention Scarpia—"A bigoted satyr and hypocrite, secretly steeped in vice, yet most demonstratively pious"—the first hint we have in the opera of the relentless character, whose desire to possess Tosca is the mainspring of the drama.
A cannon shot startles them. It is from the direction of the castle and announces the escape of a prisoner—Angelotti. Cavaradossi suggests the grounds of his villa as a place of concealment from Scarpia and his police agents, especially the old dried-up well, from which a secret passage leads to a dark vault. It can be reached by a rough path just outside the Attavanti chapel. The painter even offers to guide the fugitive. They leave hastily.
The Sacristan enters excitedly. He has great news. Word has been received that Bonaparte has been defeated. The old man now notices, however, greatly to his surprise, that the painter has gone. Acolytes, penitents, choristers, and pupils of the chapel crowd in from all directions. There is to be a "Te Deum" in honour of the victory, and at evening, in the Farnese palace, a cantata with Floria Tosca as soloist. It means extra pay for the choristers. They are jubilant.
Scarpia enters unexpectedly. He stands in a doorway. A sudden hush falls upon all. For a while they are motionless, as if spellbound. While preparations are making for the "Te Deum," Scarpia orders search made in the Attavanti chapel. He finds a fan which, from the coat-of-arms on it, he recognizes as having been left there by Angelotti's sister. A police agent also finds a basket. As he comes out with it, the Sacristan unwittingly exclaims that it is Cavaradossi's, and empty, although the painter had said that he would eat nothing. It is plain to Scarpia, who has also discovered in the Mary Magdalen of the picture the likeness to the Marchioness Attavanti, that Cavaradossi had given the basket of provisions to Angelotti, and has been an accomplice in his escape.