The BAU team heads to Long Beach when a killer obsessed with Greek mythology is murdering people with arsenic.
In each case, the unsub leaves a note for the victim to find, a few hours before they die. He also ensures that a piece of twine is left with their body.
The BAU figures out that the only thing that the victims have in common is that they all visited the same coffee shop on the same day.
It turns out that the unsub visited the coffee shop just after learning that he was dying from cancer. He would have gone in, except he saw all of the victims in the shop, looking happy. That dynamic - their happiness and his misery - triggered a childhood fantasy involving Greek gods and fate. He decided to play the part of a god, deciding their fate.
The twine that he used in each of their deaths was measured by centimeter to approximate each of their ages.
After the BAU hunted down what would have been his final victim, they realized that the unsub had found a more worthy target - his childhood friend.
His friend had prevented him from leaving for Greece twenty years ago by getting him drunk so that he missed his flight. In retaliation, he decided his friend should be the next victim.
When the BAU finally caught up to him, they informed him his friend had actually saved him from certain death all those years ago, because the bus that he was to take to the airport had an accident and all passengers were killed.