Nine hours after resting, Gideon and Harrow descend again into Canaan House's facility. Harrow explains that the earlier hatch key only granted access to experimental theorem labs, and that the Response challenge taught her how to make regenerative bone constructs. At the bottom, amid the taped outlines and dried blood from Abigail Pent and Magnus Quinn's deaths, Dulcinea Septimus appears with Protesilaus and proposes an alliance.

Dulcinea says she has solved one theorem lab and found another she cannot complete physically. Harrow initially rejects collaboration, then accepts because Palamedes Sextus might otherwise help Dulcinea and obtain the key. On the way to Laboratory Eight, Gideon helps Harrow across a broken grate, while Dulcinea quietly tells Gideon that Colum the Eighth plans to fight Protesilaus and admits the Seventh helped provoke the conflict.

Inside Laboratory Eight, marked #14-8 DIVERSION and labeled Avulsion, Dulcinea takes blood from Gideon and Harrow to pass her ward. The chamber contains a long death field between a striped line and a distant plinth. Harrow tests the barrier and finds that it withers living tissue, destroys bone, and disperses necromantic control; Dulcinea explains that senescence and entropy spells are perfectly overlaid. Dulcinea reveals Palamedes already refused the method because the field requires siphoning power from someone outside it.

Harrow steps out with Gideon and explains that she would have to drain Gideon's thalergy while crossing the field, and that Gideon's resistance would kill Harrow. Gideon agrees anyway, saying she would rather be used as a battery than have Harrow invade her mind and that agreement is enough because Harrow asked. Harrow warns that the process will be agonizing and imperfect, but Gideon remains willing.

Harrow begins the siphon and crosses the field while Gideon collapses in extreme pain, bleeding, losing sensation, and nearly dying. Dulcinea cradles Gideon, comforts her, urges her not to give herself away again, and narrates Harrow's progress as Harrow reaches the plinth, solves the mechanism, takes the key, and barely returns. The field strips away Harrow's robes, paint, hair tips, and bone adornments, leaving her exhausted and naked when she rushes back to Gideon.

After Gideon passes out and wakes in Dulcinea's arms, Harrow jealously insists that Dulcinea and Protesilaus not touch her cavalier. Harrow gives Dulcinea use of the key later but makes clear Gideon is not Dulcinea's concern. Before leaving, Harrow asks Dulcinea why she wants Lyctorhood, and Dulcinea answers that she did not want to die. Harrow and Gideon return to their rooms with the new silver key; Harrow admits she underestimated the danger, rebukes Gideon for valuing her life so cheaply, questions the purpose of the theorems, and almost reveals why she urgently needs to become a Lyctor before ordering Gideon to rest.