"The Replacement" revolves around Xander getting zapped with a magical demon ray. A big blue robe-y creature named Toth is after Buffy. The Scoobies track it to the Sunnydale dump, where Toth tries to hit Buffy with a bolt of lightning from his staff. He accidentally hits Xander instead, knocking Xan into a pile of trash. Xander seems okay, though, and the Scoobies walk him home. Little do they know that there is another Xander still lying in the trash pile. Toth's ray has split him in two. One Xander has all the confident, adult qualities. The other his nervousness and insecurities. When the Xanders meet up, both of them are sure the other is a demon. (Or possibly an evil robot.) The confident Xander enlists Buffys' help, while the insecure one winds up at Willows dorm. Both Willow and Buffy promise to help their Xander.
Xander has been trying to move out of his parents' basement and get an apartment of his own. Anya, after being injured by Harmony's goons last week in "The Real Me," is struggling with the idea of death. After 1122 years as a vengeance demon, she's scared at having as "few as 50 years left" to live. She wants Xander to make a real commitment to her. When the confident Xander signs a lease on a new apartment, he and Anya celebrate. Suddenly, the insecure Xander shows up waving a gun and trying to rescue Anya from the Xander that he's sure is a demon. Luckily, Buffy has figured out what Toth's ray really did and she and Riley get to the apartment in time to stop the Xanders from killing each other. When Toth attacks, though, Buffy has to stop him once and for all. The episode ends with Xander moving into his new place and Riley realizing that Buffy's feelings for him aren't as deep as he'd like.
There are some good parts to "The Replacement." Ironically, I think that it's Riley's best episode. I really like his reaction to the two Xander's. "Doesn't it make you wanna lock them up in separate rooms and do experiments on them?" And then at the end of the episode, he shows heretofore unknown depths by calmly declaring that Buffy is his dream woman. "But she doesn't love me." When I watch this episode, it's like I can see the character that Riley COULD be if they just stopped trying to push him as the tough guy, super cool, ultra commando, nothing-but-a-boyfriend role. If the show let him have some quirks and develop real personality, I think that the character could have worked. I also enjoy Giles confrontation with Toth. Smashing him with a statue of Offta, the goddess of childbirth, until Toth turns and "sweeps out majestically." And, I like that Anya, given the choice between the Xander's, thinks that the "adult" one is her real boyfriend. It shows that she really does see Xander in an admirable and positive light. Still, she'd like to try sleeping with both Xanders before Giles and Willow do the "reconnecting" mojo, because after all, "It's not like it would be cheating." Finally, I love the two Xander's interacting. They're both him. But, they're so different that you can easily tell them apart. It's not until the end of the episode that they start acting alike. (Laughing at "Star Trek" jokes, etc...) Giles is right, Xander really is a bad influence on himself.
On the down side, I wish Spike had been in this episode more. Also, I'm not sure why Toth was after Buffy in the first place. Just 'cause she's the Slayer or did he have some deeper plan that I just missed?
My favorite part of the episode: Willow and Xander's conversation, which references the season three episode, "Dopplegangland." Xander: "Wait 'til you have an evil twin. See how well you handle it." Willow: (pouty) "I handled it fine."