Helen pursues Adam Worth to nineteenth-century London, where she must stop him from changing history -- while also avoiding her younger self and her friends.
The Sanctuary team tries to keep the conflict between the Lotus Defense Corps forces and the Hollow Earth Abnormals from boiling over into all-out war, as more and more continue to reach the surface.
With the Sanctuary under scrutiny by a U.N. official, Magnus must explain a recent mission that went awry. Henry's werewolf girlfriend shows up with news that is sure to complicate things.
Helen travels to a remote island off the African coast to meet the Sanctuary's new financial adviser, only to find herself in the middle of a deadly hostage situation. Back home, Henry and Will search the underground for a missing Abnormal.
After coming up against Homeland Security's new task force that is hunting Abnormals, Magnus and Henry track them to a New Mexico facility — where they find that Nikola Tesla is the head of the research lab.
A pair of flying Abnormals arrive at the Sanctuary with ulterior motives, and end up appealing to Magnus for assistance. Meanwhile Will attempts to bring aid to the Sanctuary in Liberia, but a mugging leaves him blinded.
Henry joins Declan and a team from the UK Sanctuary to investigate a ship run aground in the Bering Sea, where a crew that was transporting Hollow Earth Abnormals is found to have all killed each other.
An Abnormal attack leaves Abby in an altered state, where she can only understand communication that is sung. Henry and her FBI partner Gavin retrace her steps to try and determine what happened to her.
After a spider-like device infiltrates the Sanctuary, Magnus and Tesla go inside a virtual model of the Sanctuary to try and combat a sentient Praxian computer virus.
After tangling with a monster hunter that has captured a legendary Abnormal in the caves of Bolivia, Magnus and Will are injured and trapped in a cave-in. Meanwhile, Henry records video messages meant for his future child.
Will encounters a new ally in Caleb, leader of Hollow Earth's insurgency, who hopes to create a new homeland on the surface. But he isn't prepared for the consequences of Helen's long-game plan.
After Addison and SCIU turn the new Abnormal homeland into a prison colony, Caleb plans extreme measures against the human race – while the Sanctuary network collapses, and Will and Magnus are set against each other.
