When a car careens out of control over an embankment, killing a middle-aged man in the driver's seat, Megan learns that all is not well on a pretty suburban cul-de-sac---where she and her team discover a neighbourhood swingers' club behind the picket fences and a possible toxicity in the soils among the homes. Megan also struggles between sustaining her improving relationship with Lacey and her boss Kate dating her ex-husband and seeming to interject herself into Lacey's life.

First regular appearance: Mary Mouser as Lacey Fleming.

Megan investigates the death of a young nanny run down by a car and discovers that the dead woman's 6-year-old charge has been kidnapped and requires medication. A handsome FBI agent, Derek Ames, is assigned to the case. Meanwhile, Bud comes under scrutiny when the prime kidnap suspect dies during interrogation. Megan's mother offers to help her, but Megan is leery that it's just a publicity stunt for her judicial re-election campaign. The kidnapped boy's parents hide a disturbing secret that may expose the real kidnapper.

First recurring appearance: Cliff Curtis as Derek Ames.

Recurring character: Joanna Cassidy as Joan Hunt.

A man who is presumed dead walks off Ethan's examination table, leading the team to a shocking discovery.

Peter stumbles upon a house fire and rescues a young woman. When Megan goes to the scene of the fire, she discovers that there is another victim who perished. This dismays Peter, who didn't know there was someone else in the house, and he now has a personal attachment to this case. As Megan begins her investigation, she is usurped by arson investigator Ray Easton and his colleague, Skip, who challenge her at every step. When Megan complains to Kate, she remarks that people usually find Megan impossible to work with, and the tension between the two heats up.

Final appearance: Eric Sheffer Stevens as Bill Parkson.

Recurring character: Nathalie Kelley as Dani Alvarez.

A horse-riding champion is found dead in the woods, and her convict workmate is a suspect in the ensuing murder case, as is the stable owner. Megan is starting to patch things up with Lacey. Lacey, also a rider, provides a valuable tip in the case. But Megan is about to find out the body is the proof, and they all have something to say.

A 17-year-old boy is found dead by the aftermath of an out-of-control party, and no one knows who made the 911 call. Megan's ex-husband, Todd, drops a bombshell, telling Kate, Megan and Lacey that he needs to move to California to teach at Berkeley. Lacey is faced with a difficult decision: stay with her mom, or move out west with her dad. Which one will she pick? Bud announces that his wife is pregnant. Megan and her FBI friend Derek finally get that date.

Final appearance: Cliff Curtis as Derek Ames.

When a pharmaceutical rep is murdered, with the body drained of blood, Megan and the team must find the original crime scene if they hope to find out the cause of death. The number of suspects with access to many unknown drugs poses a big hurdle.

A murdered young woman's body turns up in place of a medical school cadaver during class, leaving Megan to rely on the eager star student to help solve the case.

Walter Brown, a recent multi-million dollar winner in the lottery, is found stabbed to death in his apartment with $50,000 floating out of his window. While most of Walter's friends, including his best friend, chalked up recent behavioral changes to his newfound wealth, Megan finds an endocrine tumor that could have caused the changes. In another case, Kimberly Gleason went into the hospital for an appendectomy and died of a serious bacterial infection. Ethan has trouble finding the cause until a second patient turns up dead. With Kate's help, Ethan learns how to run an investigation and deal with grieving family members. Meanwhile, Lacey uses Megan's credit card without permission to buy a hat. Megan returns it and makes Lacey earn her own money---in a way that gives Lacey new insights into the work her mother and the team do.

After a bride plummets to her death on her wedding day, everyone from the angry ex-boyfriend to the overbearing father is a suspect. But just like a bride herself, Megan pays attention to every tiny detail until she can solve the crime---including the bride's willingness to let the wedding be stopped. Meanwhile, Ethan is falling hard for the new girl, Dani, not knowing that she's already found love with someone else on the team.

When a woman recently found innocent by Judge Joan Hunt of her child's murder is killed, Megan and her staff must find who did it. A sensationalistic reporter impedes their progress. Meanwhile, because of the controversy surrounding the case, Judge Hunt is voted out of office.

When the owner of an Italian restaurant turns up dead in the freezer, Dr. Hunt and the team take a closer look into family matters. Meanwhile, Peter must face his own haunted past.

Recurring character: Nathalie Kelley as Dani Alvarez.

After a car accident reveals a body in the trunk, the team is led to another, leaving them with two cases to solve, while Megan is reconnected with an acquaintance and Kate's job is threatened.

First recurring appearance: Jamie Bamber as Aiden Wells.

When the parents of a perfect family are murdered and discovered by their son, the team soon realizes that things are not as it seems. Meanwhile Curtis struggles with his new position, and Megan's new love life heats up.

Two young girls are involved a tragic car crash. One dies and the other is in critical condition. As Megan and her team investigate the case, however, they realize there has been an awful mistake. Not only was the wrong girl pronounced dead, but the crash may not have been the actual cause of death. Lacey faces a new medical condition.

When scores of people start dying from similar symptoms, the team is faced with a city-wide outbreak of an unknown virus and no one is safe - including Megan and her team. Center for Disease Control (CDC) Officer Charlie Stafford (guest star Luke Perry) takes over the case to figure out not just what caused the outbreak but who.

As the death toll continues to rise, the team needs to move quickly to figure out how to treat this virus and, more importantly, who is spreading it.