Copy-pasted from Wikipedia, then edited by me to give it my awesome humor.
In Soul Eater, Maka and Soul Eater are students at Death Weapon Meister Academy, (aka DWMA, aka Shibusen) as well as Black * Star, Tsubaki, Kid, Liz, and Patty. Shortly after the start of Soul Eater, Maka and Soul Eater face off against Medusa, who forces Crona and Ragnarok to fight Maka. Later on, Medusa and her cohorts attack DWMA with the intention of reviving Cthu- uhm, I mean Asura!!!! (damn, i got stop doing that). Despite the combined efforts of Maka, Black Star, and Death the Kid, Medusa's group successfully revives Asura, upon which the series, which before hand was cruising down Awesome Street, decided to stop at the nearest Nightmare Fuel Station. Medusa is seemingly killed by Stein, while Crona surrenders to DWMA and goes on to enroll there and befriend Maka.
As a result of Asura's spreading madness, Medusa's older sister Arachne comes out of hiding after 800 years. Arachne reforms her organization Arachnophobia, which poses itself as a serious threat to DWMA. Shinigami calls in death scythes from around the world to aid in the fight against Arachnophobia. During this time, Medusa resurfaces with her soul in the body of a young girl, and forms a truce with DWMA so they can annihilate the threat of Arachnophobia together. The DWMA students and Medusa's entourage infiltrate Arachnophobia's headquarters where Maka defeats Arachne, only for Medusa to betray DWMA, possessing Arachne's body and brainwashing Crona into rejoining her. Meanwhile, Death the Kid is captured by Noah, an enemy posing as the former meister 'Eibon'. Following this, Maka finally succeeds in turning Soul Eater into a death scythe. The duo become part of the newly formed meister unit Spartoi, which successfully manages to rescue Death the Kid and take down Noah.