An old prophecy foretold the birth of a pony by the name of liberal. Now that is has come true, the princesses must face the other half of the prophecy. That pony will confront an evil foreign to Equestria, one no other pony can confront.
The princesses cannot prepare for the upbringing but perhaps Liberal's cutiemark will provide with answers.
Time is running out, there are merely three fillies who have the skill to figure out the ponies cutiemark and prepare Equestria for what's about to come next.