The piece begins with a bird-like flute solo darting around the hall signifying the Holy Spirit. Its ending note is picked up by a solo choir boy.
As it begins, all of the performers are in harmony and agreement. During the course of the Mass, however, the street choir begins expressing doubts and suspicions about the necessity of God in their lives and the role of the Mass. The street chorus sings with the Latin lyrics until they hit a line which they twist into a complaint or a self-serving boast; i.e. "dona nobis pacem" (English: "grant us peace") turns into the street chorus, "Give us peace NOW!". In this way, Bernstein interweaves and contrasts social commentary and prayer.
The street chorus's bitterness and anger continues to grow and makes each of the subsequent meditations more harsh. At the play's emotional climax, the growing cacophony of the chorus' complaining finally interrupts the elevation of the Body and Blood (the transubstantiated bread and wine). The celebrant, in a furious rage, hurls the consecrated host, housed in an ornate cross-like monstrance, and the chalice, smashing them on the floor. At this sacrilege, the other cast members collapse to the ground as if dead while the Celebrant sings a solo. This solo blends the chorus's disbelief with his own crisis of faith. He feels worn out and wonders where the strength of his original faith has gone. At the end of his song, he too collapses.
A bird-like (Holy Spirit) flute solo begins again, darting here and there from different speakers in the hall, finally "alighting" in a single clear note. An altar server, who was absent during the conflict, then sings a hymn of praise to God, "Sing God a Simple Song". This restores the faith of the three choirs, who join the altar server, one by one, in his hymn of praise. They tell the Celebrant "Pax tecum" (Peace be with you), and end with a hymn asking for God's blessing. The last words of the piece are: "The Mass is ended; go in peace."