Last Wednesday's episode of "Criminal Minds" was intense and bloody as the BAU took on a case involving a killer who used medical experiments on his victims in effort to stop the aging process.

In the midseason finale, titled "Future Perfect," the FBI agents went on the hunt to find an amateur doctor obsessed with immortality who killed and threatened to kill his human guinea pigs, reports Entertainment Weekly.

Apparently, the unsub was traumatized by an elderly man at a nursing home that caused him to become fixated on finding a cure to prevent people from getting older and then dying from old age. To do that, he performed blood transfusions on his subjects and used the DNA from animals with longevity to try to create a "fountain of youth" remedy.

Eventually, the BAU tracked him down and arrested him as he was in the middle of performing a heart transplant. Unfortunately, his subject died before the BAU could reach her.

"Criminal Minds" will now go on a hiatus until mid-January. When the procedural drama returns next year, "Grumpy Cat" actress Aubrey Plaza will guest-star as Cat Adams, love interest to Dr. Spencer Reid (Matthew Gray Gubler). TVLine reports that Reid's dinner with Plaza's erratic character quickly becomes the date from hell when she pulls a gun on Reid. This transforms into a game of wits when Cat discloses that she knows that he works for the FBI and then tasks him to think on his feet to save himself and the other people in the restaurant. The "Parks and Recreation" alum will guest-star in the Season 11 midseason premiere on Jan. 13.

Episode 11 of Season 11 of "Criminal Minds" will premiere on Wednesday, Jan. 13 at 9 p.m. ET on CBS.