Pete's helping out a diner in Pittsburgh, PA by eating as much of their pie as he can before the power outage causes it to spoil, but Myka's more concerned with finding out why so much of the city can't keep their fridges on. A powerful blast from an unknown source took out a huge chunk of a billboard and killed a man working on it, and though most people think it was just lightning, the worker's reduction to a pile of dust tells Myka that something more sinister is at work.
The method of destruction rings a bell for Myka, but she hesitates to tell Pete, knowing he won't like the connection she's made: Roughly one hundred years ago, when Britain ruled Warehouse 12, an agent named Helena Wells investigated a case just like this. Against Pete's wishes, Myka proposes - and the Regents approve - that H.G. Wells, the woman who almost destroyed the planet out of grief for her dead daughter, be brought on to consult on the case. Despite the fact that she's only present in holographic form and her presence is controlled through a black sphere which Myka holds, Pete is livid at the thought of working with the woman who almost made Myka quit the Warehouse.
But he must swallow his anger as Helena fills them in on her investigation in the 1890s. Her colleague, with whom she was building a rocket, had been pulverized in the exact same manner as the billboard worker. This rocket used, as a key element, the Horn of Joshua, a trumpet that once brought down the walls of Jericho, and is now destroying Pittsburgh. When she realized the havoc the Horn could wreak, Helena launched the rocket into space, thinking to rid the earth of its destructive powers forever - but it seems the rocket came back to Earth.
Claudia triangulates the locations of the pulverizing events - now ratcheted up to three - and finds they were all aimed at Gliese 581 g, a celestial body believed to be capable of sustaining life, and all emanated from the defunct SETI (Search for Extra-Terrestrial Intelligence) center in Pittsburgh. If someone aims the horn at Gliese 581 g today, Claudia calculates, it will rip right through Alleghany Field, where some 40,000 fans are gathered for a ball game.
Just then, Artie remembers another incident with the Horn, involving two American agents in Ohio in 1962. Several objects were destroyed by the Horn, and one life was lost. The victim's son, a man named Daniel Varley, is a SETI member living in Pittsburgh. Myka, Pete and the holographic Helena rush to the run-down SETI center, where Daniel is preparing another blast, using the horn in hopes of contacting the aliens who he believes abducted his father. Pete and Myka are unable to persuade him to cancel the mission, but when Helena appeals to him, telling her own sad, unfulfilled story of seeking revenge for the death of her daughter, Daniel's heart is penetrated. He stops the countdown and turns the horn over to the agents.
Thousands of lives were saved, and Pete has to acknowledge the at-times baffling complexity of human psychology, while Myka says goodbye - again - to a woman who could've been a great friend and teammate.