Due to an accident involving mirror Makes-Things's experiments, five EPC agents were sent to the prime multiverse, while four PPC agents went into the mirror. The EPC agents were neutralized, while one of the PPC agents was turned into a sleeper agent. With the help of EPC dissenters, the PPC agents managed to return to their own continuum.
The EPC traitor Ontic escaped with the PPC agents and would join the PPC proper. She spied on the League of Mary Sue Factories and briefly returned to her home multiverse to raise a group of Daleks to defend the PPC during the 2008 Mary Sue Invasion.
It is theorised that the Protectors of the Plot Discontinuum stories also take place somewhere in the mirror multiverse, making the PPD an AU version of the EPC, while neither are technically AUs of the PPC proper, being part of a separate multiverse entirely.