The city is in a panic as 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 Don brings in to help out.

The episode was rated 7.76 from 211 votes.

While consulting with the NTSB at the scene of the wreck, Don learns that this tragedy is one of a series of recreated accidents involving railroad negligence. As is the pattern with the previous train wrecks, an undecipherable numerical code is left at the site, which Don recruits Charlie to solve. By breaking the code, Charlie is able to establish that this accident was a recreation of a deadly wreck that left only one survivor several years ago. Don's investigation leads him and his team to a dangerous standoff with their suspect, who has surprising ties to the railway system.

The episode was rated 7.76 from 217 votes.

The team attempts to find the daughter of a geneticist who they suspect was kidnapped by the unstable mother. However, they become concerned about the case when they find evidence that suggests the young girl was a clone. Meanwhile, Charlie and Amita discuss having kids and Liz reveals a dark secret.

The episode was rated 7.77 from 111 votes.

Various people in the L.A. area with seemingly nothing in common start to become extremely sick and die on the same day. Don fears bioterrorists may have released a deadly virus into the environment. While he tries to discover whether anyone is behind the outbreak, Charlie tries to locate the point of origin.

The episode was rated 7.77 from 245 votes.

Don talks to his therapist again, and while he's gone, the team races to find an eleven year-old boy being held on a $3 million ransom, and time is running out...

The episode was rated 7.77 from 124 votes.

Megan feels responsible for an agent's death following an explosion at a meth house the FBI had targeted for a bust. When the kingpin eludes capture, it's discovered he may also be involved in child pornography when one of the photographs on his computer turns out to be a young girl in a sexually explicit position. The picture sends Charlie on a crusade of his own, as he remembers a girl he knew as a young child that he suspected was being abused, and he's determined to save the little girl in the picture.

The episode was rated 7.79 from 158 votes.

Don investigates the murders of two young women found in their cars outside their homes. Their deaths were made to look like drug overdoses, but Don realizes a serial killer is responsible. Meanwhile, Charlie is having bizarre dreams about his parents. In one dream, his deceased mother appears and tells him he has a question to ask her, but he can't imagine what it could be.

The episode was rated 7.80 from 138 votes.

When an assailant opens fire in the FBI offices, Don and his team must determine his motive and possible connection to a dangerous arms dealer who is on trial. After nearly getting shot, Charlie is shaken and reluctant to return to the offices, which hinders his contribution to the case.

The episode was rated 7.82 from 144 votes.

After four people nearly die from poisoning, Don learns someone is tampering with nonprescription drugs. When the perpetrator publishes a manifesto in a local paper, Don is led to believe the person may be a former employee of the drug company that manufactured the drug.

The episode was rated 7.83 from 163 votes.

When Don and his team are called to the scene of the murder of a Los Angeles gang member, they learn they are investigating the murder of another agent who had been working undercover.

The episode was rated 7.84 from 158 votes.

To help capture a serial rapist-turned-killer, FBI Special Agent Don Eppes recruits his genius brother Charlie, who uses a mathematical equation to identify the killer's point of origin by working back from the crime scene locations.

The episode was rated 7.87 from 361 votes.

In the season finale, Don and Charlie work together on a case that ends up with them going head to head about their personal beliefs, and one team member will walk away forever.

The episode was rated 7.88 from 117 votes.

After Charlie and Amita get married and begin preparations for their London adventure, they put a slight hold on their wedded bliss when they assist the team in helping Don track down the whereabouts of his gun after it goes missing.

The episode was rated 7.88 from 113 votes.

Feeling guilty over the stabbing, Charlie puts his attention on finding a serial killer using math theories to help the team solve the case before another life is taken.

The episode was rated 7.92 from 107 votes.

Don's life hangs in the balance after he is injured during a raid while investigating a series of home invasions.

The episode was rated 7.93 from 114 votes.