Resident Evil 4 was perhaps the most awaited portion of the Resident Evil franchise. It expanded upon the concepts that were introduced in its predecessors, while weaving in plot and content from other spin-off games. However, for such a monumental work, Resident Evil 4 is known for being the longest (so far) in terms of development. It was started in 1999, and went through a total of 4 iterations, before its release in 2005.
The Series picks up with Leon Kennedy (one of the survivors of the Racoon City Incident, and one of the primary protagonists in Resident Evil 2), who is now a member of the US Secret Service. Leon is assigned, upon taking the job, the not-so-enviable task of recovering the Daughter of the President, who has gone missing.
Leon ends up in Spain, where he uncovers "Los Plagas", a viral disease that is turning the inhabitants of a small remote agricultural town into slaves to some unknown will. They seek to kill him, and prevent him from recovering the President's Daughter. Their ultimate goal, as they are controlled by an organization known as 'Los Illuminados', is to infect her with 'Plagas' and return her to the United States to see to the infection of government officials, and thereby gain an unstoppable power base in the States.