Kouichi Uehara is a fourth grade student living in the suburb of Tokyo. One day, he picks up a large stone which turns out to be a fossil of a baby "Kappa" who has been sleeping underground for the past 300 years, and names it "Coo." They become good friends and Coo starts living with Kouichi's family. However, Coo has a hard time adjusting to life among humans and misses his kind.

Aikawa Kazuhiko was the captain of Tendoji high school prestigious basketball team. He moves into a new town to attend Mizuho high school and joins its basketball team. However, Mizuho high's basketball team is far from being prestigious, in fact, it's now defunct. Nevertheless to say, Kazuhiko's persistence, passion and basketball skills inspired other team members of the dysfunctional basketball team to gear up and start practicing again. The goal is to play in the national tournaments where all young basketball players meet their opponents to compete with them. The tale of youth of the five protagonists: Fujiwara Takumi, Ishii Tsutomu, Dobashi Kenji, Miura Ranmaru and Aikawa Kazuhiko have just began along with the live of Mizuho high school basketball team.

One year ago, a UFO containing 150 aliens crash-landed off the shores of Kasai. Because no one could fix their ship, the Japanese Government decided to bestow upon them the designation "DearS" and make them into Japanese citizens, teaching them the language, customs, and culture of Japan. However, in order for them to become more familiar with human society, a home-stay program has been enacted to allow them to mingle with other humans. One misty morning, a truck carrying a capsule that housed one of these aliens ends up dropping it into the riverbank, releasing her from her confinement. She is eventually found by a high school student named Takeya Ikuhara, who saves her from being hit by a truck and takes pity on her, despite being extremely distrustful of their race and wanting nothing to do with them. Upon being named Ren, she imprints upon him as her "Master" and serves as his personal "Slave," leaving him with a "DearS" who wants to remain with him no matter what and bringing his ordinary, alien-free days to an end.

Tokyo-born schoolboy Yukitaka Tsutsui is moving to Yamagata Prefecture for high school on a baseball scholarship. Since he went to the top middle school in Japan for baseball, the townsfolk are very excited about his arrival. However, when he arrives in his apartment, he encounters a strange man nonchalantly reading and wearing his clothes! The stranger claims to be an alien who crash-landed on Earth and has nowhere to go. Revealing himself to be Baka Ki El Dogra, the crown prince of the planet Dogra, he is just one of the hundreds of aliens that have already made Earth their home. Despite his regal origins, the prince is an infamous intergalactic fool who thinks nothing of inconveniencing others for his own amusement. Whether he is running ridiculous tests on his subordinates, giving strange powers to random children, or just generally being a nuisance, nobody is safe from the idiot prince's antics!