Well we will find out as TVLine has learnt exclusively.
The details of the alternative reality episode, set to be the shows sixth episode of the new season, is still under wraps but star Jon Huertas (Detective Javier Esposito) says it's his favourite episode of the upcoming episodes.
“The first couple [episodes] can’t bring too much comedy, but this ‘alternate universe’ is classic Castle, where you mix a great procedural crime with comedy.”
Huertas reveals that the episode will be “a completely different world” where all of the major characters we love still exist but in different ways.
“It was a lot of fun, getting into that script and seeing how we might all be a bit different. It’s a really fun episode, and people are really going to enjoy it. It’s one of those episodes where you have to laugh.”
Of course this isn't the first alternative reality episode on TV. Shows like Friends, Buffy, Grey's Anatomy and more recently, The Big Bang Theory have all done 'what if' episodes.
Can Castle pull it off?
Castle will return to ABC on September 29.