Nina sets criteria for a "replacement" for Andrew: younger, beautiful, desperate, and appealing to Andrew’s tastes. With Enzo’s reluctant help, she hires a private investigator who confirms Wilhelmina “Millie” Calloway’s desperation and background, prompting Nina to offer Millie the live-in job immediately.

Anticipating Andrew’s resistance to a live-in, Nina fabricates incapacity with migraines and deliberately lets the house deteriorate. She places Millie in the attic to associate Millie with Nina’s role and to entice Andrew. Although Andrew initially objects, he relents when he experiences a spotless home, a good dinner, and Millie’s presence.

Nina then engineers the three conditions she needs: mutual attraction, Millie’s hatred of Nina, and opportunity. She lets herself go, treats Millie cruelly, and stages a meltdown over fabricated missing PTA notes after trashing the kitchen, ensuring Andy comforts Millie and sees her as the wronged party. This deepens their connection and isolates Nina as the antagonist.

To escalate, Nina weaponizes Andrew’s desire for a baby. She secretly maintains an IUD and, using incriminating photos obtained by the investigator, coerces Dr. Gelman into declaring her unable to carry to term. She primed Evelyn with a faux pregnancy call to stir family expectations, then provokes Andrew into a dramatic fight, smashing a mirror that Millie overhears, cementing Millie’s belief that Nina is unstable. Finally, Nina ensures opportunity by arranging Showdown tickets and an overnight—meant for Millie and Andrew—and sends Cecelia to camp to remove leverage. Tracking Millie’s phone to Manhattan confirms the affair. Seeing their changed looks afterward, Nina concludes Andrew is in love with Millie and considers herself free.