This post contains frank discussion of Season 7, Episode 4 of Game of Thrones: “The Spoils of War.” If you’re not caught up or don’t want to be spoiled, now would be the time to leave. Seriously, I won’t warn you again. Skedaddle.

This week’s episode of Game of Thrones featured an impressive, much-hyped, blockbuster battle between Jaime Lannister’s men and Daenerys’s dragons and Dothraki. But, surprisingly, that wasn’t the most fascinating showdown of the night. Up north, a petite assassin and a lady warrior stole the martial-arts spotlight as fans were treated to a Winterfell brawl between Arya Stark and Brienne of Tarth. The sparring match was compelling because the various blocks, parries, and thrusts carried the weight of one of the season’s most important themes.

When Brienne and Arya end their little duel in a truce of sorts, Brienne asks, in astonishment, “Who taught you to do that?” “No one,” Arya responds, a wry callback to her training in the House of Black and White. But the question of who taught Arya to become the dark little killer she is today is vital for understanding the future of the Starks.

All season long, Thrones has been exploring the influence of the various non-Starks who had a hand in raising the orphaned Arya, Sansa, Bran, and Jon. Bran, of course, has transformed utterly into his mentor: the Three-Eyed Raven. That loss of all Stark identity is his tragedy. Meanwhile, Sansa is struggling to balance the lessons of duty and honor learned from her mother and father with the manipulative influence of both her mentors: Littlefinger and Cersei. “You almost sound like you admire Cersei,” Jon remarks in Episode 1. “I learned a lot from her,” Sansa admits with a tinge of regret.

But none of the Stark children had as many adoptive parental figures as Arya Stark, who has, since she lost her father in Season 1, been bounced from one dark father to the next. “Spoils of War” makes much of Arya’s carefully curated list of enemies. This habit of vengeful list-making is something she picked up from one of her least damaged babysitters, Yoren, in Season 2.