Ah, the famous After You've Gone. Or "Since Your Parting" in the new Viz translation.
This chapter is basically a turning point for Ataru. Whereas he previously would've loved getting rid of Lum, now he'd actually miss her.
That doesn't mean that he will give up on girl-hunting. But he started growing an attachment to Lum.
The way both anime adapted this chapter is fascinating. Both of them made the original story of the manga way more dramatic—especially episode 44 of the original anime.
The original Episode 44 was voted as the no. 1 Urusei Yatsura episode twice. I'm pretty sure Episode 10B of the Remake will also be voted as such in the November 2023 poll just to keep with tradition.