Mission: Impossible is an American television series that chronicles the missions of a team of secret American government agents known as the Impossible Missions Force (IMF). In the first season, the IMF leader is Dan Briggs, played by Steven Hill, then in the second season, the team gets a new leader: Jim Phelps, played by Peter Graves. He remains the IMF leader until the end of the series. With a few exceptions, the formula for each Mission: Impossible episode is the same each week for the first 4 seasons. Beginning with Season 5, an prologue was added to each episode.