Elissa’s story concerns fortune, which sometimes brings comeuppance to those who blame others for sins that they share. Sister Isabetta, a young Lombard nun, has fallen in love with a young man. He has contrived a way to allow them to spend nights together in her cell. One night, when other nuns see him leaving, they decide to lie in wait and catch Isabetta in the act rather than telling the Abbess, Abbess Usimbalda, right away.
Elissa uses her free-choice tale to continue the theme of trickster wives from Day 7 but also to offer a counterpoint to the amorous young monk of Dioneo’s first tale (I, 4). In featuring sexually uninhibited nuns, the tale also recalls the nuns who become Masetto’s personal harem in III, 1. The other nuns’ eagerness not just to tell on Isabetta, but to get her into trouble by catching her in the act, suggests jealousy and spitefulness unbecoming of nuns—but characteristic of anticlerical satire.
The nuns keep a careful watch, and the next time Sister Isabetta is with her lover, they run to Abbess Usimbalda’s room. But she is with her own lover, and dressing quickly in the dark, she accidentally puts his undergarments on her head instead of her veil. The other nuns don’t notice at first, because they are so focused on catching Isabetta in the act. When they do find Isabetta with her lover, the nuns carry her to the chapter-house, where the Abbess scolds her sharply.
Usimbalda and her priest-lover embody clerical hypocrisy that is much worse than Isabetta’s youthful dalliances. Putting her lover’s underwear on instead of her own headdress makes her sins impossible to ignore and makes her look foolish in front of the nuns for whom she’s supposed to set a good example. The other nuns’ impatience again suggests eagerness to witness Isabetta’s punishment rather than a legitimate concern for her spiritual or moral wellbeing.
Sister Isabetta silently endures Abbess Usimbalda’s abuse for a while, until she notices her odd headwear. Then she daringly suggests that Usimbalda “tie up [her] bonnet” before chastising others. A somewhat confused Usimbalda continues scolding, but when Isabetta repeats herself, the Abbess and the other nuns finally realize that she’s wearing breeches as a veil. The Abbess changes her tune, now “arguing that it [is] impossible to defend oneself against the goadings of the flesh.” Sister Isabetta and Abbess Usimbalda continue to see their lovers, despite the envy of the other nuns who, single, must “[console] themselves in secret as best they [can].”
Isabetta gets away with having a lover of her own, because her abbess is guilty of the same sin. This parallels the earlier stories where the young Tuscan monk and his abbot share the country girl (I, 4) and where Masetto finds himself in bed with all the nuns and the abbess (III, 1). It also emphasizes the themes of clerical and feminine hypocrisy. In the first case, as soon as she realizes she’s caught, Usimbalda disingenuously changes her tune; in the second, like Pietro’s wife (V, 10), she chastises someone else for the same sins of which she herself is guilty.