The first game explains that "Splinter Cell" refers to an elite recon-type unit of single covert operatives (such as Sam Fisher) who are supported in the field by a high-tech remote team.
In the first three games (Splinter Cell, Pandora Tomorrow, Chaos Theory), terrorists are planning attacks, usually by use of information warfare, which Sam Fisher, an operative for Third Echelon, a secret branch of the NSA, must prevent. The missions range from gathering intelligence to capturing and/or eliminating terrorists.
In the fourth game, Double Agent, Fisher assumes the identity of a wanted criminal in order to infiltrate a terrorist ring.
The fifth game, Conviction, starts after events in the conclusion of Double Agent: Sam has abandoned Third Echelon. When he discovers that the death of his daughter Sarah had not been an accident (as had been purported at the beginning of Double Agent), he strikes out on his own in search of those responsible until Sam's investigation uncovers a conspiracy within his old agency.
In the sixth and most recent game, Blacklist, Third Echelon has been disbanded by the President of the United States. A new outfit, Fourth Echelon, is formed by the President and placed under the command of Sam Fisher with the mission of stopping the Blacklist attacks and the Engineers - the organization behind them. Blacklist deals with the morality of war and how far Sam and his team go in order to prevent these plots against America.
