As the last storyteller on the subject of love stories ending in tragedy (as the final storyteller, Dioneo, takes the liberty of straying from the day's theme), Filostrato recounts the tale of Guiglielmo Guardastagno and Guiglielmo Rossiglione, two noble knights and close friends. Guardastagno eventually falls in love with Rossiglione's wife and begins to meet secretly with her. When Rossiglione discovers that both his wife and his best friend are betraying him, he springs an attack upon an unsuspecting Guardastagno and rips his heart out of his chest. Later that evening, he arranges for his cook to prepare the heart in a tasty stew to be served to his own wife. After his wife has enjoyed the dish to its fullest, Rossiglione vengefully tells her what she has just eaten. In despair, she throws herself from an open window and falls to her death.
Filostrato's tale skillfully incorporates many of the same elements found in the story told by Fiammetta earlier in the day (IV.1) but in a manner that inverts the events and outcomes. Most notably, whereas Ghismonda controls the events in Fiammetta's story, in Filostrato's story control rests squarely in the hands of Rossiglione. It is Rossiglione, the wronged lover, who plots and carries out Guardastagno's murder, and though he, like Tancredi, chooses to rip out the heart, the symbolic seat of love, he goes one step further and has the organ "minced" - therefore completely destroyed. Moreover, he arranges for his wife to participate unwittingly in the organ's destruction by serving it to her as a meal.
One of the most notable contrasts between the two stories is the opposing images of the raw and the cooked hearts. In particular, the cooked heart in Filostrato's story lacks the "curative" properties imbued in the raw heart in Fiammetta's story. Instead of providing the wife with a medium for a sort of redemption, the cooked and eaten heart serves only to shock and disgust her. "God forbid that any other food should pass my lips now that I have partaken of such excellent fare as the heart of so gallant and courteous a knight as Guiglielmo Guardastagno," she cries. Despite her noble words, there remains a vulgar aspect in her having not only eaten, but devoured her lover's heart with relish, "finding it so tasty a dish that she ate every scrap of it." Boccaccio suggests a sullying of her love for Guardastagno, and thereby calls into question its noble nature.
In Filostrato's story Rossiglione, the husband, is clearly the central character. The wife hardly has a voice in the narrative. Aside from praising the tastiness of her meal, she does not speak at all, much less plot and reason as does Ghismonda in her lengthy and eloquent soliloquies. Filostrato does not even reveal the lady's name, as if her individuality were insignificant to the story. She is "any woman", not any particular woman, and her fate, in Filostrato's construction, can be read as the fate of "any woman" who succumbs to similar circumstances. His view is decidedly misogynistic in that it figures the woman as the voiceless, nameless catalyst behind a man's tragic undoing. The wife is guilty of the double transgression against marriage and friendship. In Filostrato's account Rossiglione is therefore both more justified and more successful than Tancredi in exacting revenge for the expression of misbegotten love. Although Rossiglione's wife, like Ghismonda, commits suicide, she does so without intending for the action to honor and defend her love for Guardastagno. Her leap from the window is not a premeditated act, but an impetuous action born in distress, for it is "without a second thought that she allowed herself to fall."
Filostrato makes of point of telling the listeners that Rossiglione's wife was "not only killed ... but almost completely disfigured" in the fall, as if purposefully drawing a stark contrast to Princess Ghismonda who laid herself to rest quietly upon her bed to die, adorned in all her finery. In Filostrato's story, with the lover's heart having been ground and eaten and the beloved's body smashed to pieces and destroyed, the lovers' deaths offer none of the romantic transcendence so apparent in Fiammetta's story. It is, in fact, gruesome. Although interred together as were Ghismonda and Guiscardo, Guardastagno and Rossiglione's wife evoke more pity than praise as the townsfolk gather the parts of their dismembered and defiled bodies to lay them to rest. Their tombstone "bore an inscription, in verse, to indicate who was buried there and the manner and the cause of their deaths," as if to serve as a warning to others who may be tempted to follow in their example.
The link between Fiammetta's (IV.1) and Filostrato's (IV.9) stories is reinforced through the use of the narrative frame. At the close of the day Filostrato cedes his crown to Fiammetta who reigns over the next day's proceedings.