Sayonara Zetsubou Sensei OVA.

Jealous of Suneo's new robot toy, Nobita asks Doraemon to build him an even better one. Doraemon initially refuses, until Nobita accidentally discovers pieces of a mysterious robot that falls from the sky. After gathering all the robot parts and assembled them together, the giant robot, Zanda Claus, is soon completed. The duo soon learn that the robot is not a mere toy, but a powerful weapon in the fight against the coming Robot Army that is going to attack Earth and enslave the human inhabitants of it. An invasion is near, as a mysterious girl Riruru (リルル, Alternative spelling: Lilulu, Lillele [Doko Demo Doa Scanlations]) shows up, looking for the robot. (Source: Wikipedia)

Under the cover of darkness, a masked samurai murders six men across the metropolis of Japan: three in Tokyo, one in Osaka, and the last in Kyoto. In their investigation, the police learn that each man was a member of the Genjibotaru—a thieves gang centered on the theft of Buddhist statues and artifacts and who go by the names of Minomoto no Yoshitune's servants.

Without a clear motive or clues to the other members' identities, the case runs dry until a Kyoto temple calls for the famous Kogorou Mouri. Having received an anonymous letter containing a peculiar puzzle, the temple monks ask for his assistance in solving it to recover their long lost statue. Meanwhile, Conan Edogawa and high school detective Heiji Hattori team up in order to solve the cryptic puzzle and find the murderer, as Hattori searches for his childhood love.

With Hattori's knowledge of Kyoto, the two scour the streets and gradually discover the truth, but not before the murderer strikes again—killing another Genjibotaru member and, after repeated attempts on Hattori's life, eventually kidnapping Hattori's childhood sweetheart. It is only by working together to bring buried clues to light can Conan and Hattori hope to end the rogue samurai's bloodshed and save Hattori's love.

[Written by MAL Rewrite]

On the surface, Koudo Ikusei Senior High School is a utopia. The students enjoy an unparalleled amount of freedom, and it is ranked highly in Japan. However, the reality is less than ideal. Four classes, A through D, are ranked in order of merit, and only the top classes receive favorable treatment. Kiyotaka Ayanokouji is a student of Class D, where the school dumps its worst. There he meets the unsociable Suzune Horikita, who believes she was placed in Class D by mistake and desires to climb all the way to Class A, and the seemingly amicable class idol Kikyou Kushida, whose aim is to make as many friends as possible. While class membership is permanent, class rankings are not; students in lower ranked classes can rise in rankings if they score better than those in the top ones. Additionally, in Class D, there are no bars on what methods can be used to get ahead. In this cutthroat school, can they prevail against the odds and reach the top?