Don and Charlie investigate the murder of a judge's wife who was gunned down in her garage. Since she was driving her husband's car, it's unclear whether she was the intended target---or her husband, who is currently hearing a death-penalty case involving a local gang leader. As the probe intensifies, Don reconnects with an attractive prosecutor whose life may also be in danger.

A woman gives a Beverly Hills jewelry-store owner a note informing him that his wife and daughter have been kidnapped and will die unless he hands over diamonds worth thousands of dollars. He gives them to her, but then she is gunned down by a security guard. The shooting gets Don involved in the case and he enlists Charlie's help in identifying the dead woman through her car's keyless remote.

A stalker frightens a popular singer in her Hollywood Hills house---but doesn't harm her. The FBI becomes involved when she reveals a series of threatening letters she's received through the mail. Meanwhile, Charlie analyzes the handwriting from the letters and learns that they are from two different people. Later, a photographer is found dead near the singer's house, and Don believes the man was murdered and that his death is connected to the stalking case.

The star witness in a federal stock-fraud case against the CEO of a now bankrupt major corporation is gunned down in her house before she can testify. The victim was the company's chief financial officer and her testimony was likely to bring down the CEO and other corporate executives. The only witness to the murder is the woman's 11-year-old son who is in shock and is reluctant to talk.

Don and Charlie try to unravel an assassination plot to be carried out on U.S. soil. They get wind of the plan after a forger is arrested and Charlie decodes a notebook found in the forger's apartment. The investigation leads to the son of a slain Colombian leader.

A toxic gas is released on a Los Angeles subway during an antiterrorism drill run by the Department of Homeland Security, and Don must determine whether it was an accident or a legitimate attack before more drills are carried out over the next few days in various locations across the U.S. Back at home, old wounds are reopened between Don and Charlie when they learn that a girl they fought over in high school is going to use their dad's house for her wedding.

Don investigates a series of home invasions in which the thieves steal only high-end items from wealthy individuals. In the latest robbery, a father is killed trying to protect his family. Meanwhile, Charlie becomes upset when a rival theoretician (Colin Hanks) challenges his most famous work and states that it is flawed. Back at home, Alan prepares to open his own consulting business.

Megan feels responsible for an agent's death following an explosion at a house where methamphetamine is being illegally produced and that the FBI targeted for a bust. While several men are arrested in the raid, the kingpin gets away and it's discovered that he may also be involved in child pornography. When Charlie learns this and sees a disturbing picture of a young girl at the raid site, he becomes obsessed with finding her.

Don learns that someone is poisoning non-prescription drugs made by a leading pharmaceutical company after four people nearly die from the tampering. The perpetrator then publishes a manifesto in a local paper that leads Don to believe that the person may be a former employee of the drug company. Later, Larry shares his theory of the case with Megan and he discovers that he's attracted to her.

A woman researcher who specializes in Native American antiquities is attacked and killed at the museum where she works. The museum is located on federal land so the FBI is called in. Don's investigation reveals that the woman was working late at night on a secret project that involved the carbon dating of an object. Back at home, Don and Charlie become worried about their dad after they learn that he is still holding on to his late wife's clothes, which he supposedly gave away months ago.