Growing up, acting mature... Those terms have never seemed to apply to Pinkie Pie, not in the slightest.

When Twilight, irritated with her friend's random behavior, mentions them during their argument, Pinkie's reaction is far beyond what she could have expected. As is Rainbow's rather violent attempt to defend the earth pony.
Driven by guilt—and a tinge of curiosity—Twilight asks the pegasus for help in understanding the enigma that Pinkie Pie is.

What she is to discover, is bound to change her view of her earth pony friend in ways she could have never anticipated.