A serial rapist and killer terrorizes L.A., and Charlie attempts to pinpoint the suspect's point of origin by using a mathematical equation based on the various crime-scene locations. Kate: Karen Norby. Haldane: William O'Leary.

Charlie accurately predicts where a band of bank robbers will strike next, but when Don and his team confront them a massive shoot-out occurs and four people, including an officer, are killed. The outcome disturbs Charlie and he retreats into the family garage to work on an unsolvable math problem that he also plunged into after his mother become ill a year earlier. But Don needs his brother's help and tries to get Charlie to return to the case.

Various people in the L.A. area with seemingly nothing in common become extremely sick and die on the same day, and Don fears that bioterrorists may have released a deadly virus into the environment. While he tries to discover whether anyone is behind the outbreak, Charlie attempts to locate the point of origin. Claudia: Elizabeth Connors. Havercamp: CCH Pounder. Kramer: Max Van Ville. Weaver: J.K. Simmons. Jessica: Marita Geraghty.

Charlie believes that a college student who allegedly committed suicide by jumping from a bridge was actually murdered, and that his death is related to an engineering thesis he was working on about one of L.A.'s newest and most important buildings that may not be as structurally safe as its owner claims it is. Back at home, Alan prepares for his first date in 35 years.

A 6-year-old girl is kidnapped by a pair of clowns hired to perform at her birthday party shortly after the celebration ends. The girl's dad (Neil Patrick Harris) is a brilliant mathematician who is working on a complex problem and refuses to cooperate with the police, saying that he can get his daughter back without their help. This leads Charlie to believe that the kidnappers are after something that the missing girl's father is working on. Back at home, Alan decides to sell his house.

A serial saboteur claims responsibility for a deadly accident in which a train crashes into an empty school bus that is abandoned on railroad tracks. The perpetrator leaves a note that appears to be a numeric code, which Charlie tries to crack. Soon it's learned that the accident is a re-creation of a previous wreck, and that the person behind it is responsible for six train mishaps over the past three years, all of which were copies of earlier rail disasters.

Don learns that counterfeiters are at work producing small-denomination bills, and that they have taken an artist hostage to draw the images for the fake money. If the missing woman isn't located soon, Don fears that she will be killed when her work is finished on the phony cash. The counterfeiters have already murdered at least five people, two of whom stole money from them. The case is made more difficult for Don when his former lover, now a Secret Service agent, is assigned to it.

A man wanted for stock fraud is found garroted in his apartment and the crime is eerily similar to a murder committed a year earlier, a case Don closed when an ex-con confessed. Now, Don reinvestigates the old case to determine if he put an innocent man in jail. He asks Charlie to go over the evidence to see if anything was missed the first time around. Kim: Sarah Wayne Callies. Blanchard: Vince Deadrick Sr. Cummings: James Leo Ryan. Eddie: Victor Raider-Wexler. Karen: Dale Dickey.

The city is in a panic after a sniper goes on a shooting spree and randomly kills several people, including a postal worker. The investigation reveals that more than one shooter is at work. As Charlie works the case, he's frustrated by a sniper expert (Lou Diamond Phillips) Don brings in to help out. Walter: Don Fischer. Susan: Arabella Field. Marion: Jayne Taini. Osborne: Franc Ross. Nathan: Nathanael Johnson. Big Mama: Heather Lee.

A truck carrying radioactive material is stolen, and the thieves threaten to set off a dirty bomb in L.A. in 12 hours if they aren't paid $20 million. While Don attempts to track down the truck, Charlie tries to come up with the most likely location where the bomb may be detonated to inflict the most damage to the population. Gerth: Reynaldo Rosales. Peter: Ron Dean. Mark: Jack Howard. Fitchman: James Parks. Vicky: Carrie Preston. Sites: Victor Rivers.