Zoe releases Inspector Harriet Marblemaw from the guesthouse and, with Arthur’s help, stages a “demonstration” from Phee: an orange rapidly becomes a full fruit tree, knocking Marblemaw down. Marblemaw responds by weaponizing her “courtesy points” and trying to pit the children against each other, but the household keeps to its plan of unsettling her without outright harming her.

Over the next days, Marblemaw probes for weaknesses and attempts to assert control. Theodore leads her on a three-hour “tour” that is really a fly chase, earning more sarcastic “courtesy points.” At supper, Marblemaw insults Talia’s beard and gender presentation, prompting Lucy to magically give Marblemaw a handlebar mustache that becomes effectively permanent. During class observations, Marblemaw repeatedly interrupts; Arthur pointedly lets her know he’s aware she contacts Rowder via a special briefcase, and the class ends the moment she begins preaching.

Marblemaw continues antagonizing the family and is repeatedly embarrassed. In the garden, she again presses Talia to shave; Talia digs a large hole and bluntly warns it will be Marblemaw’s grave if she brings it up again, driving the inspector away in fury. At supper, Chauncey’s excitement over a single armpit hair ends with him scream-sneezing ink all over Marblemaw, leaving a stark outline on the wall. On Thursday, Marblemaw tries to interfere when Sal shifts into his small dog form to play with Calliope; Calliope attacks Marblemaw and drives her outside, and Arthur needles Marblemaw by suggesting rabies shots, which she refuses because it could endanger Calliope.

On Friday, Arthur and Zoe finally meet with Marblemaw; Zoe privately shares a secret plan with Arthur that stuns him, though they keep it from Marblemaw. Lucy interrupts the meeting covered in mud and leads everyone outside to show he has created three six-foot sentient mud people (Janet, Barry, and Turnip). Marblemaw calls the act blasphemous and accuses Arthur of failing as a father; Arthur answers that he already is one and then publicly tenders his resignation as master for DICOMY, severing his official tie to the Department.

When Marblemaw demands “honesty,” Sal steps forward and dismantles her claim to know what’s best by listing intimate facts about each child and confronting his own past abuse in orphanages. Marblemaw threatens they will return “in greater numbers,” so Arthur ends her visit immediately. Zoe commands Turnip to carry Marblemaw to the docks, where Zoe formally banishes her from Marsyas Island with a shell made from Turnip’s mud, warning any return will cause unimaginable pain; Marblemaw leaves down the salt road. Lucy releases the mud people to live in the forest, and they pledge loyalty to Zoe as “your majesty,” asking to remain as Marsyas’s mud representatives—hinting they recognize Zoe’s true status. As they head home, Lucy suggests they finally abandon the word “master.”