A mad god owed him money.
Mister Lewis was just trying to do his job. He “consulted” on problems of a supernatural nature. Unfortunately, while uncovering why all the musicians were suddenly dying, how a cult was able to pay off their student loans through blood sacrifice and who was using a ghost for industrial espionage purposes, the god of tricksters left him holding an unpaid bill.
So as Mister Lewis traverses the country, doing unpleasant work for unpleasant people, he keeps an eye peeled for Loki.
Some debts need collecting.