This masque in classical verse is a paean to the power and rewards of virtue in the face of temptation. Jove sends an attendant Spirit to defend three noble siblings lost in a wild wood after darkness has fallen. The Spirit favorably contrasts the "starry threshold of Jove's court" with the snares of earthly life. It relates how Bacchus and the witch Circe gave birth to a son, Comus, who surpasses his mother in the black arts. He entices mortals to drink of his cup promising sensual pleasure, but they are transformed into beast-headed people who forget their former lives and, unaware of their animal countenances, think themselves more attractive than before.

Comus appears leading his monstrous band of revelers and praises the night that provides cover for their sensual pleasures. He hears the lost Lady coming and transforms himself into a shepherd to more easily ensnare her. She has overheard their revels and is wary. She trusts that a virtuous mind and her chastity will protect her, and that the "Supreme good" would "send a glistering Guardian if need were." She sings a song to the nymph Echo to attract her lost brothers' attention if they are in earshot.

Comus detects something divine in her song and compares it to the singing of the Sirens. They meet and converse. The Lady explains her predicament and Comus, in disguise, offers to help but only as a ruse to gain her trust so he can try to ensnare and corrupt her. She trusts him and follows. Meanwhile, the brothers are worried about their lost sister, but the elder brother tells the younger that their sister's virtue will protect her. The younger brother doubts, but the elder urges hope and that their sister has a hidden strength in chastity that evil cannot overpower. They encounter the attendant Spirit in the guise of their royal father's shepherd. He tells them of Comus and that the sorcerer has already captured their sister. The elder brother says he'll make Comus return his sister or slay him, but the Spirit warns him that the magician can easily "unthred" his "joynts" with his wand. The Spirit in disguise tells them he has a potent herb that can protect all of them.

In his stately palace Comus tries to persuade the Lady to drink of his glass and experience sensual pleasure. He threatens to turn her to marble with his wand if she declines but she replies she will still have control of her mind. Comus argues that nature gave her fine limbs to put to good use. She relates his previous deceptions and rebukes him. He regales her with a speech about how nature bestows beauty for a purpose, not to have it hidden and neglected. She rebukes him again with such a powerful speech that Comus is unsettled but recovers his nerve and persists.

The brothers and the Spirit in disguise burst onto the scene and repel Comus and his enchanted monsters but need help to free the Lady from his spell. They call on the water Nymph Sabrina who is a protectress of virtue. Sabrina appears and the Spirit requests she release the chaste lady from the enchantment that holds her in the sorcerer's chair. Sabrina complies and leaves and the Spirit praises her. The Spirit then urges them to leave the cursed palace and return to their father's castle.

Back in Ludlow Castle (where the Mask was first performed in 1634) dancers accompany the Spirit, the Lady, and her two brothers. The Spirit signals the end of the play, and the children are presented to their father and mother and praised for their victory over "sensual Folly and Intemperance". The dances end and the Spirit closes with a poetic eulogy to many of the classical divinities of the eternal realm and urges that mortals should follow virtue if they would rise, and that heaven itself bends down to help the virtuous when virtue is endangered.