The episode follows Billy Maximoff (Joe Locke) as he pieces together the details of his origins and continues his journey on the Witches' Road alongside Agatha Harkness (Kathryn Hahn).

In 2023, 13-year old William Kaplan celebrates his bar mitzvah in Eastview, New Jersey, where the fortune teller Lilia Calderu reads his palm and tells him his lifeline is split in two, and seeing something else she doesn't disclose.

[2] By October 2021, a "dark comedy" spin-off from WandaVision centered on Kathryn Hahn as Agatha was in early development for Disney+ from Marvel Studios,[3] with Schaeffer returning as head writer and executive producer.

[10] When discussing Evan Peters' appearance as Ralph Bohner, Schaeffer explained the character, along with other Westview residents, was meant to embody "the regular human experience" of supernatural conflicts.

"[10] The episode stars Kathryn Hahn as Agatha Harkness, Joe Locke as Billy Maximoff and William Kaplan, Sasheer Zamata as Jennifer Kale, Ali Ahn as Alice Wu-Gulliver, Debra Jo Rupp as Sharon Davis, and Patti LuPone as Lilia Calderu.

[11] Showrunner Jac Schaeffer expressed excitement about Evan Peters reprising his role from WandaVision, calling his return "a prerequisite" in her decision to develop the follow-up series.

[12] Kathryn Hahn and Locke improvised parts of their interrogation scene, including the pen-clicking gesture and Agatha Harkness' challenge to Maximoff to "poke the bear."

[16] Nielsen Media Research, which records streaming viewership on U.S. television screens, estimated that the series was viewed for 410 million minutes for the episode's premiere week.

[17] JustWatch, a guide to streaming content with access to data from more than 40 million users around the world, reported it as the eighth most-streamed series in the U.S. for the week ending October 20.

"[19] CBR critic Joshua Patton rated the episode 7/10, praising Locke and Hahn's comedic scenes and finding it "hilarious to see their dynamic from a sillier and more self-aware angle".

Gates lauded Locke and Hahn's chemistry, stating that the two "continue to be excellent scene partners, equally engaging in their dramatic and comedic moments", but expressed mixed feelings on the episode's pacing, calling it "a double-edged sword" to develop Billy Maximoff's backstory while not offering a similar look into other members of Agatha Harkness' coven.

[21] Caroline Framke of Vulture praised director Gandja Monteiro's approach, which "immediately achieves the balancing act of keeping "Familiar by Thy Side" close enough to the show's witchy world while rooting it in the very human story at the heart of Billy's.

[23] Joshua Yehl of IGN felt the episode offered payoff on the series' ongoing mysteries, calling it "a well-executed character study that brought together many of the show's story threads in a cool way".