Alan Eppes has two sons. Don is the dedicated Los Angeles FBI agent. His younger brother, Charlie (David Krumholtz), is the brilliant mathematician. The two brothers couldn't be more different, but they find a common ground when collaborating to solve complicated crimes in Numb3rs. Don is also assisted by fellow agents Megan Reeves, David Sinclair, and Colby Granger , while Charlie finds insight and encouragement from a co-worker, Dr. Larry Fleinhardt (Peter MacNicol), and Amita Ramanujan, Charlie's former grad student. Season Four of Numb3rs brings Don and Charlie together on perplexing cases ranging from a mass poisoning at a cult compound to a death connected to the secret world of reality games.