"Cause and Effect" is the fifteenth episode of the sixth season of Numb3rs.
Charlie and Amita are getting married at last. While making an arrest, Don's gun is stolen from him and is subsequently used in a series of crimes. Lou Diamond Phillips' character, Agent Ian Edgerton, appears in the series finale, along with many other popular recurring characters to round things off.
Title
One of the most fundamental precepts of physics - a chain of causality. A number of key storylines are thus resolved, major plot elements laid peacefully to rest.