Derek 'Ladies Man' Morgan runs into a woman in the coffee shop and, as he often does, begins to flirt with her. He is therefore surprised when she knows who he is. Turns out that that is because she is to be JJ's temporary replacement at the BAU while she (JJ) goes off to have her baby.
The BAU are called to California where a series of brutal homicides are taking place along Highway 99. People are being murdered in their homes after which the unsub sticks around and wears their clothes, eats their food and sleeps in their beds. Given the fact that the crimes are taking place over a radius of more than 400 miles, the team profile that the killer travels as a part of his work. When it is found that travelling by train is the link, the team learn more about why the crimes are being committed and what the causal link is in time to move in to stop him.