Nero is a young demon hunter who lives on the isolated island of Fortuna and is also a member of the Order of the Sword: a religious sect of knights who worship the Legendary Dark Knight Sparda as a God. Dante arrives, and murders The Order's leader, the High Priest Sanctus, in front of the entire congregation. At the same time, an army of demons invade the city, putting everyone, including Nero's love interest Kyrie, in danger. Tasked with stopping Dante by Kyrie's brother Credo, the Captain of the Holy Knights, Nero's journey leads him to discover that he is in fact a descendant of Sparda himself, and Dante is not his enemy.
Gameplay in Devil May Cry 4 is like previous games in the series. The player must fight through levels called "missions", occasionally solving puzzles or gathering items. Performance in a mission is graded from "D", the lowest grade, to "A" then "S", "SS", and "SSS", the highest grade. Grades are based on items used, Red Orbs gathered, time taken, and the number of Style Points accumulated. Each Style Point grade has its own tag-word. The stylish grade shows up on the side of the screen and begins at "Deadly" (D); progresses through "Carnage" (C), "Brutal" (B), and "Atomic" (A); then, advances through one last bar of grades containing the phrases "Smokin'" (S), "Smokin' Style" (SS), and lastly "Smokin' Sick Style" (SSS).
Source: Wikipedia, "Devil_May_Cry_4," available under the CC-BY-SA License.