Henry has finally cracked through SCIU's security system to discover their latest plunders. One, in Bolivia, catches Magnus's interest - it seems SCIU is after Quetzalcoatl, the winged serpent of legend, a beast that Gregory Magnus searched long and hard for. One attempt came shockingly close, but when Gregory returned from the expedition as the only survivor, he strictly forbade any further pursuance.
Magnus and Will rush to the SCIU site, where they find a team of scientists and thugs attempting to transport the deadly beast. Magnus tries to reason with their leader, but the serpent is soon free of its bindings and, in a flash, kills most of the SCIU team and strands Magnus and Will deep within a cave. Will was shot in the stomach and Magnus fears she may have broken ribs, but Magnus manages to find water and dress Will's wound. Within moments, both are recovering miraculously from their injuries - could it be that the legendary powers of healing and wisdom bequested by Quetzalcoatl are real?
Back at the Sanctuary, Henry is making videos for his son, with the bittersweet intention that he be able to watch them after Henry's death. With the Hollow Earth revolution heating up, Henry's not sure he'll be there to be a father to his son and wants little Alistair to know something about his parents - a gift that Henry never had.
Will and Magnus find what could be a path to the surface, but Will's moods are swinging wildly and he barely has the will to go on. Magnus realizes that the water they drank, while temporarily giving them strength, is then sapping it and bringing a bad crash. In this low, Will vents all his feelings of resentment and anger at Magnus's firebrand nature - how it almost killed Abby, and how she takes his one, mortal life for granted because of her indefinite lifespan. She is hurt, but recognizes this venom as the effect of the water.
One more turn through the caves, and they find themselves face to face again with the serpent. Will is weak and Magnus is reticent to let him drink again, but petulant and tired of taking orders from her, Will grabs the magic water and downs it all. Suddenly he gains superhuman abilities, and is able to throw rocks that splinter the cave and allow him and Magnus to slip past the beast. But the crash after such a high almost ends his life, and Magnus must return to the serpent's spring to refresh their water supply. When she does, she realizes that the gemstones near the spring are the true source of the strange abilities. She gathers a few for study back at the Sanctuary.
When they finally return home, Will and Magnus discuss the heated words exchanged in the bowels of the earth. Magnus tries to communicate that all she ever did, she did for the greater good. Will understands, but insists that he be given greater weight in the decision-making process: He wants to be Magnus's partner. And, he wants to know more about this grand plan she's been keeping so secret. In her true, cryptic style, she pauses to consider, but cannot yet let Will in to that world.