Spoiler - TV Guide

Loved the Castle premiere, but what was Castle doing with a half-naked woman in the promos for the next episode?

ADAM: As creator Andrew W. Marlowe already teased, that encounter is a consequence of Castle pretending to be single in order to keep his relationship with Beckett a secret from their co-workers. But that doesn't mean infidelity won't be an issue. An upcoming case will implicate Castle in the murder of an ex. "It makes everybody question: How well do we actually know this guy?" Marlowe says. But it gets worse: "Castle may have been having an affair with her at the same time he was dating Beckett," he teases. "That goes to the heart of all of Beckett's insecurities."

Fonte: TV Guide