After a conversation with the Mane Six causes Spike's mind to wander and fill with worries, he tries to find a quiet spot to collect his thoughts and think things thru. However, during his "free time", he finds himself in the magic mirror room again. As he watches himself, he makes a simple, silly mistake that ends up sending him back to that strange, pony-less world. This time, he doesn't have Twilight's help, but the strangest thing is the way he appears there: much less dog-like and much more...human.
(This story is located in a "flexible" timeline, the only events that occurred in the human world were the ones of the first movie.)