Owen, Michelle and the 126 crew race to a rattlesnake infestation at a young family's home, and attempt to rescue a man with dementia from a dangerous scene that he created. Meanwhile, Owen adopts a firehouse dog with cancer and Michelle finally learns the truth behind her sister's disappearance. Also, a tragic accident shakes the team to its core.

Without telling Emily or her parents anything, Bill decides with Zoe to test on Emily the biological weapon he hopes to be able to use against the aliens. Catherine, for her part, has had difficulty managing her grief since Mokrani's death. She now has only one thing on her mind: to find out who Bill Ward is and find him. Sophia is ready to help him, but Nathan hesitates.

Catherine, Sophia and Nathan make their way to London, accompanied by Victor, a survivor encountered by chance. Along the way, the dangers multiply. Sacha's feelings towards Emily intensify and the young man's behavior becomes more and more disturbing. Feeling threatened, Bill is forced to flee.

Super villain M.O.D.O.K. runs his evil organization AIM into the ground and is forced to sell it to the tech company GRUMBL. As the megalomaniacal M.O.D.O.K. struggles to regain control of AIM, he risks losing something even more important…his family!

Without telling Emily or her parents anything, Bill decides with Zoe to test on Emily the biological weapon he hopes to be able to use against the aliens. Catherine, for her part, has had difficulty managing her grief since Mokrani's death. She now has only one thing on her mind: to find out who Bill Ward is and find him. Sophia is ready to help him, but Nathan hesitates.

Catherine, Sophia and Nathan make their way to London, accompanied by Victor, a survivor encountered by chance. Along the way, the dangers multiply. Sacha's feelings towards Emily intensify and the young man's behavior becomes more and more disturbing. Feeling threatened, Bill is forced to flee.

When former BAU team member and current Interpol Agent Emily Prentiss tracks an international serial killer, she enlists the help of her friends at the BAU after she is convinced the UnSub's next victim is on American soil. Also, the BAU deals with Morgan's departure from the team.

Beckett and Castle are followed by a documentary crew as they investigate the murder of a rock star.

The BAU searches for an UnSub who is purposely disfiguring victims. Also, Rossi has an awkward reunion with his ex-wife, Hayden Montgomery, who kept his daughter, Joy, a secret from him for nearly 30 years.

In the aftermath of a disastrous "Meet the Parents" dinner, a bickering Castle and Beckett are sent to retrieve the only witness to a murder.

R.J. unravels the truth about a Little League coach whose family is being mysteriously targeted; R.J.'s parole officer worries that he is pushing the boundaries of his conditional release.

Owen and the team race to deal with a runaway bull at a used car lot and an apparent electrocution at the hands of an LA transplant. Meanwhile, Owen is determined to prove he is physically fit for the job while undergoing chemo and Michelle gets the strongest lead yet on her missing sister.

The team investigates the death of the leader of the Lynwood University male acapella singing group, "The Whippersnaps," whose body was found in the school rat lab. The victim recently kicked out Ian Jansen, one of the group's best singers (guest star Jordan Fisher), sending Booth and Aubrey to question Ian and other current members of the male group (including guest stars Mitch Grassi and Scott Hoying) and some stars of the competing female acapella group (including guest star Kristin Maldonado) to get a handle on the group dynamics and what could have led to the murder of one of their own. Meanwhile, surprising information about Aubrey's own college experience is brought to light, Hodgins applies for an experimental nerve regeneration study, Booth tries to plan a summer trip with Parker (guest star Gavin MacIntosh) and a new intern joins the Jeffersonian lab (guest star B.K. Cannon).

When former BAU team member and current Interpol Agent Emily Prentiss tracks an international serial killer, she enlists the help of her friends at the BAU after she is convinced the UnSub's next victim is on American soil. Also, the BAU deals with Morgan's departure from the team.

The BAU searches for an UnSub who is purposely disfiguring victims. Also, Rossi has an awkward reunion with his ex-wife, Hayden Montgomery, who kept his daughter, Joy, a secret from him for nearly 30 years.

Coping with a death that strikes close to home by focusing on work, Harrow is stunned to discover a connection between this death and another deceased man on his autopsy table.

R.J. unravels the truth about a Little League coach whose family is being mysteriously targeted; R.J.'s parole officer worries that he is pushing the boundaries of his conditional release.

When a bearded, rotund man in a red Santa suit with an ID reading Kris Kringle falls from the sky in Central Park, Castle and Beckett are on the hunt to find the person who killed Santa.

A drunk college student discovers the dead body of a Secret Service agent just before a presidential visit to the area. Booth is ready to hit the ground running and turns to Secret Service agent and former colleague Brandt Walker for information, but is surprised to learn the Secret Service has reservations about Booth's involvement in the case. The team uncovers some shocking evidence that heightens the seriousness of the case, causing Booth to step up for his country. Meanwhile, Brennan is out sick with the flu and former intern Dr. Colin Fisher returns to the Jeffersonian to run lead on the case, but Brennan refuses to be kept out of the loop. Also, Hodgins takes his life into his own hands and turns a corner after a potentially dangerous experiment.

When a bearded, rotund man in a red Santa suit with an ID reading Kris Kringle falls from the sky in Central Park, Castle and Beckett are on the hunt to find the person who killed Santa.

Owen, Michelle and the 126 crew race to a rattlesnake infestation at a young family's home, and attempt to rescue a man with dementia from a dangerous scene that he created. Meanwhile, Owen adopts a firehouse dog with cancer and Michelle finally learns the truth behind her sister's disappearance. Also, a tragic accident shakes the team to its core.

The BAU enters into a dangerous game of cat-and-mouse with a convicted serial killer after guards at a Virginia prison intercept a package with evidence relating to the kidnapping of two boys several years ago. The team believes the notorious killer may have information on the boys' whereabouts.

Hotch is apprehended for suspected conspiracy and the BAU suspects a larger plot may be brewing.

Harrow and Fairley are sent to a double-fatality in Fairley's hometown - a trip that will test their already-strained relationship.

A homicide detective relocates to the sleepy, middle of nowhere town of Palm Glade, looking for a change of pace from his old life with the Chicago Police Department. He soon learns this place isn’t so idyllic, as he keeps finding people murdered.

R.J. unravels the truth about a Little League coach whose family is being mysteriously targeted; R.J.'s parole officer worries that he is pushing the boundaries of his conditional release.

When a bearded, rotund man in a red Santa suit with an ID reading Kris Kringle falls from the sky in Central Park, Castle and Beckett are on the hunt to find the person who killed Santa.

Harrow and Fairley are sent to a double-fatality in Fairley's hometown - a trip that will test their already-strained relationship.

With Cam's reputation and career on the line, the team travels from overseas to help her solve the case of an unidentified young boy; the Forensics Anthropology Unit lost much of its vigor after disbanding to pursue personal ventures, and it will take the collaborative energy and knowledge of the newly reunited team to unravel evidence, and find closure for the family of the unknown boy.

Joins the BAU team, which is tasked with capturing a killer who escaped prison with 13 other convicts at the end of last season.

Owen and the 126 crew race to a gender reveal party gone horribly wrong, and a father/son trapped in a cave during a spelunking excursion. Meanwhile, Owen helps T.K. reconcile the past to help his future, Judd and Grace aid Judd's aging father and Michelle learns the full truth about her missing sister, Iris.

Castle and Beckett enter the world of adult entertainment when the founder of college girls gone crazy is murdered.

When JJ arrives home shaken after a rough case, she breaks down and tells her husband about the abduction of two children.