There is only one thing on Earth more powerful than evil, and that’s us,” Buffy Summers says at the end of “Bring on the Night” (7-10), verbalizing the theme that has held Buffy the Vampire Slayer together from its inception. Who are “us”? By the end of the series, Buffy, Willow, Xander, Giles, Anya, Dawn, and Spike, with the extended family of the surviving Slayers, Faith, Principal Wood, and even Andrew. The first four have been part of “us” from the beginning, while the others have replaced some of the original members of this odd little self-made family.

In his commentary on “Welcome to the Hellmouth,” (1-1), on the first season DVD release, producer Joss Whedon says: “The idea of this band of . . . outcasts being the heart of the show and . . . creating their own little family is . . . the mission statement of the show.” The theme of a chosen rather than blood-related family develops throughout the first six seasons of the series, until it risks destruction in the final episode of the sixth season, “Grave” (6-22). Yet family is what saves the world at the end of season six and, perhaps more than ever before, the redemptive power of that family is shown in the seventh-season events that follow. The final episode even has the title “Chosen” (7-22), as Buffy’s chosen family defeat the final assault of evil in Sunnydale.

At the end of the sixth season, Willow, in a paroxysm of  …