Receiving word that a large group of Abnormals has been spotted near a South Dakotan cave entrance, Magnus realizes it's the first wave of Abnormals emerging from Hollow Earth.

A mysterious abnormal attacks Abby, and Magnus and Will discover there's only one way to communicate with this unusual enemy - through music.

Magnus is approached by a notorious leader in Hollow Earth who is looking for assistance in building a new home on the surface for his insurgents, but problems arise when the world's government gets involved. Addison reactivates Will's federal commission.

With the new Abnormal Homeland now openly under threat, Henry and Tesla learn that the SCIU weaponized Tesla's work to use against Abnormals. Bigfoot and Kate investigate Caleb's rise to power while Will and Abby look into Magnus' past.