By Rumiko Takahashi. Released in Japan by Shogakukan, serialized in the magazine Shonen Sunday. Released in North America by Viz.
For those following along with Viz (as you should be), this whole volume can be found in the collection The Return of Lum*Urusei Yatsura Vol. 2: Lum in the Sun, as chapters 3-13.
Generally speaking it's hard to review the gag humor of a Takahashi manga. Describing most of the gags tends to ruin them for the potential reader, and a lot of them also depend on humor that makes no sense out of context, or that is already adapted from impossible-to-translate Japanese into an English equivalent. Suffice it to say that this is a very funny volume of UY. The characters are now all settled into their roles, and react just the way that we expect them to. Ataru is lustful, Mendo is egotistical, Lum is happy / angry (delete where applicable), and Shinobu is melancholy / angry (delete where applicable). We do see Shinobu's super strength debut here, as she lifts a boat (including the people in it) to hit Mendou over the head after he slights her. The manga never bothered to explain it, though the 3rd movie hinted it was due to Lum's continued presence, sort of a leaking of superpowers.
This volume can essentially be divided into two parts. The first half takes place at the school, and involves a new school nurse: Sakura, who now debuts as a full-fledged regular after occasional appearances in previous volumes. Her eccentricity comes up a few times - at first Ataru wants nothing to do with her, knowing that she tends to cause him bad luck - but after a while she settles into your standard young woman fending off sexist teenagers with her fists while occasionally using her spiritual powers to battle the insane maladies that crop up at the school. Her temper's also far worse than it was before - on par with the other women now - most likely due to being treated like a sex object all the time by Ataru and his friends.
We also meet Tobimaro Mizunokoji for the first time, Mendou's childhood friend and rival - well, OK, just rival. Tobimaro is obsessed with baseball, and determined to win a game over his longtime adversary, even after the previous 11 years have ended in 11 draws. He's pretty much a big goof, thinking only of baseball and living out in the mountains, sort of Tarzan-like. His other big notable trait is that he's one of the only male cast members not obsessed with women - he thinks of baseball as a man's challenge, and resents Lum for being part of Mendou's team. (Lum soon lets him know what she thinks of that opinion - violently.) Notably, when we see him in the other story in this volume, he has a bodyguard squad composed entirely of hot young women - whom he treats like... a bodyguard squad. Mendou would never be trusted with such a thing - his bodyguards are notably male. In any case, Tobimaro will pop up throughout the series, usually either in a baseball context or, later on, involving his insane family. He's generally more sympathetic than Mendou - though not here, where he's just a pure dumb goofball, stealing Lum's bikinis as, well, the plot requires it.
The second half of the manga moves from the school into summer vacation, and takes place entirely on the beach. This not only allows the girls in the cast to wear gorgeous swimsuits (Mendou notes that Lum's one-piece is incredibly sexy, likely as it's a change from her usual bikini - this is what prompts Shinobu, wearing a cute bikini herself, to try to hit him with a boat), but gives us some plotlines that wouldn't really work in classes, such as Cherry getting mistaken for a swimsuit-lovin' octopus, or Cherry teaching Ataru and Mendou how to surf, or Cherry teaching everyone yoga so that they can lose weight after sitting around the beach eating all vacation. Oh, did I mention Cherry's also a regular now? Yes, with Sakura's arrival comes Cherry, everyone's least-favorite Buddhist monk, who had also made occasional appearances but shows up far more often now. The series would be far less funny and far less annoying without him, so it balances out well.
Highlights of this volume include the baseball match (especially Mendou's bodyguards, who really test the limits of "how stupid can they be" here), Ataru accidentally getting combined with Mendou's physics notes, the whole ludicrous chase after Tobimaro and his bodyguards to rescue Lum's "only good clothes", and possibly the best of all, Ataru, Cherry and Sakura participating in an all-you-can-eat competition. Ataru drops out pretty early, and Cherry a bit after that, but Sakura not only eats the entire hotel out of every single bit of food they have - including an entire roast COW - but still looks slim and gorgeous. Of course, as we find when she tries to lay on a float device in the hotel pool, looks can be deceiving. Sakura's bottomless appetite will come up a few times in the future.
Having settled into her basic premise, Takahashi is now moving the pieces around, seeing what works and what doesn't. Hence Sakura and Cherry's appearances becoming more frequent. Tobimaro is also a refreshing change of pace (his constant obsession with rivalry may remind Ranma readers of Ryouga Hibiki), though she wouldn't really get a good handle on him till later. Most of all, though, she's really getting the hang of a constant stream of gags, developing into outright chaos until everything collapses right at the last page. There's only one or two chapters here that aren't as funny as they could be (the legend of the "Red Cloak" needed far more room than just 16 pages to be really good). That's a high average for a harem comedy. And next time, in Volume 6, we'll see another regular character debut - possibly the first "yandere" in all of manga and anime.