Magical Girl Lyrical Nanoha is an anime following the adventures of the titular Takamichi Nanoha and her friends. There are three seperate seasons, each with their own story arc: the original MGLN, covering the Jewel Seeds; MGLN A's covering the Book of Darkness; and StrikerS, set a decade later (Nanoha starts her career as a magical girl at the age of nine) facing Jail Scaglietti.
Nanoha's adventures usually take place on the behest of the Time Space Administration Bureau, a paramilitary force that polices the handling of magical artifacts both with a large army of mages and a fleet of magical inter-dimensional battleships. Although Nanoha and her friends are at first only loosely associated with the TSAB, as an adult they are highly regarded officers of the organisation.
Unlike many magical girl animes, MGLN has very few monsters of the week since such minor flunkies would be annihilated in short order by the powerful mages. Typically, the greatest challenge involves determining the motivations of the opponents, which are not always adversarial. In both the first two series, initial opponents are ultimately won over, sometimes by judicious use of extreme force ('befriending' as fans put it).