As the fourth season of the hit crime series opens, FBI Agent Don Eppes (Rob Morrow) and his math genius brother (David Krumholtz) continue dealing with the fallout from events that transpired in last season's finale. Some information the bureau has about mole Colby Granger (Dylan Bruno) may be erroneous, and the hunt is on when he escapes from prison. Peter MacNicol returns as a series regular, while guest stars this year include Val Kilmer.