FBI agent Jamie Pratt investigates a series of murders spanning a period of forty years — all committed with the same gun.
As it turns out, Dr. Givens has used a property of developing fetal human brains to create a time machine and has been traveling back through time to kill condemned serial killers before they strike, all to prepare herself for one final journey to stop the man who had kidnapped and raped her for five days when she was a teenager, the event having left her scarred all her life.
She assists in stopping the kidnapping and rescues the young Theresa, but the elder Dr. Givens is fatally wounded and dies.
But without that trauma, Dr. Givens was never motivated to travel back through time and preemptively murder known serial killers, one of whom eventually murdered a close friend of Agent Pratt (at the beginning of the episode this friend was dead, then alive later in the episode thanks to a temporal excursion by Dr. Givens, now dead again).
The episode closes with Agent Pratt traveling back to 1980 and shooting the serial killer who would murder her best friend in the future.