Sam and Dean are more than a little surprised when Sam's imaginary childhood friend, Sully (Nate Torrence) appears to them both in The Bunker, with the news that a fellow "imaginary friend", Sparkle (Everrett Shea), has been brutally murdered.
It turns out that a child's imaginary friend is sometimes a fairy-like creature known as a Zână, whose job is to protect and guide the child.
Sully grants the Winchesters the power to see fellow Zână, and their investigation begins.
Although ultimately the central plot of Just My Imagination is slightly contrived, the script by Jenny Klein has enough heart and sweetness to ride over those bumps.
Throughout the episode we also see flashbacks to Sully's friendship with young Sam (Dylan Kingwell) when he was left alone while Dean and their dad went on hunts.
This builds to their 'break-up' once Sam is finally allowed to to join the family business, despite Sully's insistence that there are alternatives to that lifestyle.
While a quirky murder mystery on the surface, with the usual Supernatural status quo flipped so the victims are the "monsters" for a change, the story is more about Sam questioning his decisions and seeking guidance on what he should do about the apparent "messages from God" he has been receiving.
Just My Imagination manages to squeeze in references to the season arc and concludes with Sam, in the wake of the visions he's been getting, seeming to decide what he needs to do in respect to Lucifer and The Cage.