Henry has finally cracked through SCIU's security system to discover their latest plunders. One, in Bolivia, catches Magnus's interest - it seems the agency is after Quetzalcoatl, the winged serpent of legend, a beast that Helen's father, Gregory, searched long and hard for. One expedition came close, but when he returned as the only survivor, Gregory strictly forbade any further pursuit of the creature.
Magnus and Will rush to the SCIU site, where they find a team of scientists and soldiers attempting to transport the deadly beast. Magnus attempts to reason with the SCIU team leader, but the serpent is soon free and, in a flash, kills most of the SCIU team, stranding Magnus and Will deep within the caverns. Will has been shot in the stomach and Magnus fears that she may have broken ribs, but she manages to find water and dress Will's wound. Within moments, both are recovering miraculously from their injuries - could it be that the legendary healing powers and wisdom attributed to 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 for his son and wants little Alistair to know something about his parents - a gift that he never had. Meanwhile, Will and Magnus find what could be a path to the surface, but Will's moods are swinging wildly and he barely has the motivation to go on. Magnus realizes that the water they drank, which temporarily gave them strength, is now sapping that strength. "Crashing" emotionally, Will vents all his pent-up angry resentment at Magnus's firebrand nature - how it almost killed Abby and led to the death of Druitt and Helen's daughter, Ashley - and how she takes his one, mortal life for granted because of her indefinite lifespan. Magnus is hurt, but recognizes Will's venom as an effect of the water.
After wandering through the caves, they find themselves face to face with the serpent once again. Will is weak and Magnus is reluctant to let him drink more of the "magic" water. But, petulant and tired of taking orders, Will grabs the mystical fluid and downs it all. Gaining superhuman strength, he is able to throw rocks at the ceiling, splintering openings in the cave and letting in light - allowing the two of them to slip past the beast. But the resultant 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 and gathers a few for study back at the Sanctuary.
Safely back home, Will and Magnus discuss the heated words exchanged in the dark bowels of the earth. Magnus tries to explain that all she ever did, she did for the greater good. Will says he understands, but again insists that he be given greater weight in the decision-making process - as a full partner - and he demands to know more about this secret grand plan she's been harboring. In her usual cryptic style, Magnus agrees to carefully consider Will's proposal - but she is reticent to let Will into that private world.
[Episode Recap edited from SyFy website]