The whole thing starts because the main character, Naoya, is straightforward and sincere to the point that an ICBM has less impact when it hits a wall. After confessing to his childhood friend Saki every month for the last ten years, he has finally gotten her (always was going to be given but she felt shy) consent to a relationship. Then the first wrench is thrown in when Nagisa confesses to Naoya, revealing how dedicated she was to that moment. Being a sucker for hard work and sincerity, Naoya falls for her, and rather than engage in infidelity or secrecy, goes directly to Saki and asks for her permission to two-time her. He gets punched in the face, but Naoya manages to overcome her objection via that same straightforward sincerity that made Saki and Nagisa fall for him in the first place.
Does it end at two-timing? ....no. The whole thing is over the top comedy, and it being limited to just two would limit the quantity of jokes, which follow a fairly reliable formula. Under the thick layer of comedy, the manga is surprisingly considerate of the characters and their feelings. Saki is, both in her eyes and those of Naoya and the others, the "legal wife" here: this entire situation continues at her say-so and hers alone. She can at any time terminate it and a handful of time threatens it to force the other girls to work harder for the benefit of the entire harem. Nagisa, the first girl added, is so considerate and tender that despite the situation, Saki instantly befriends her. Really, if Nagisa had not been the first girl, the whole situation may never have occurred. The other two, Rika and Shino, have competing goals: Rika wants to destroy the harem and dethrone Saki, taking Naoya for herself alone, while Saki's best friend Shino wants to either destroy the harem for the mutual benefit of Saki and herself or (and only with Saki's express permission and blessing) join as Naoya's third girlfriend.
With these contradictory motives, there also comes the problem of not being someone's number one. Late in the manga, the frustration the girls have quietly expressed is thrust out into the open, as the problem in a harem is that time is not infinite, each one has a limited amount of time to be Naoya's focus of attention, and the girls must sit on the sidelines when someone else is the focus of all Naoya's attention and not inconsiderable affection. Counter-intuitively, the girls develop friendships and relationships outside of the one they form revolving around Naoya - while he is the center spoke, they all connect to each other as well, so the entire relationship also means being thoughtful and considerate of your friends who happen to share your boyfriend.
Make no mistake, this is first and foremost a romcom. It's about the humor, but there are moments of tension and drama to prevent it from becoming rote. I recommend reading it, as it's a fun way to enjoy yourself with a silly read.
