I guess no one consulted USA when making the episode, because it's been over half a month since Halloween and the show returned only last week. Still, the episode is perfectly fine even if it isn't in season and doesn't solely rely on Halloween.
Coming off of Shawn and Juliet hooking up at the end of last episode, "In Plain Fright" deals with their progressing relationship while delivering a spooky case. It's an average episode with the usual investigations and jokes at spooky Halloween set. Along the way, Shawn tells Gus about his relationship with Juliet and he's fine with it. On the other hand, no one else in the department knows yet, which should cause troubles later on. By finally pulling the trigger on the relationship, the writers finally gave themselves a chance to push the boundaries beyond the creative stagnation that's been going on for years.