Near Warehouse 12 in London, Sir James Eddington is walking through the streets when he runs into the person he was going to meet at the observatory. There’s a burst of noise and Eddington is disintegrated.
In Greenbury, Ohio, bird-watcher Doug Varley spots a blue jay near a barn. He sees wreckage in the barn and approaches, and is disintegrated following a burst of noise.
In Pittsburgh, a man working on a billboard is disintegrated. Twelve hours later, Pete and Myka investigate. Pete flirts with the nearby waitress, Pammy, and learns that the power went out after a horn sound echoed through the streets. Myka has investigated the billboard and determined that the edges were worn like it was in a sand storm.
Back at the warehouse, Pete and Myka meet with Artie and Claudia. Artie informs them that the Regents have already approved Myka’s idea and activates a hologram sphere. An image of Helena appears and she tells them that she knows what artifact is involved. They explain to Pete that she’s an artifact but he’s still not happy that they’re working with a former super villain. Myka explains that she read the Warehouse 12 files and realized that Helena worked on a similar case in 1893. Steve is on personal leave and Artie tells Helena to stay away from him. He has confirmed that several people in Pennsylvania have disappeared accompanied by the noise. There’s no connection and Myka insists that they have to know what Helena knows. Artie warns Helena that once she’s done helping them, she’ll go back to prison.
In 1893, Helena watches on as her brothers Charles hold court with the admirers as he takes the credit for her works. Fellow warehouse agent Vincent Crowley is there and notes that people aren’t aware of what she can do. Helena’s partner William Wolcott arrives and tells them that there is a stolen artifact, and two agents have already died. Vincent says that he should go since he has more experience, but Helena insists that William will be fine. Helena goes to her room to change and takes William along. He avoids watching and tells her the details. There has been a strange dust at the scene of the murder, the same as the two dead agents. Helena considers her diary briefly and then equips herself from a hidden compartment.
The two agents investigate the crime scene and Helena finds a scrap of paper confirming that the victim was Sir Eddington, an astronomer. She explains to Wolcott that she was working with Eddington on a project and romantically involved with him. The man with the horn wanted to stop Eddington from arriving at the observatory.
In the present, Artie confirms that the horn in question is Joshua’s Trumpet, the Biblical artifact that pulverizes anything in its path. Pete ignores Helena’s input and insists they can find it on their own. Artie realizes that there’s something else familiar about the case and checks the files. Helena informs Claudia the Trumpet has a range of 15-20 meters and Pete points out that the man in the billboard was further away. Myka speculates that whoever has the Trumpet has augmented its power and asks what happened to it.
In 1893, Helena and William go to the observatory where a rocket is stored. Helena explains that she helped build it and confirms that the Trumpet has been placed in the rocket as a power source. She talks about her diary and explains that the thief learned about the rocket and hired two men to capture them. The two men come up and capture them and take them to their leader... Vincent. As he prepares the rocket for launch, Vincent explains that he plans to use the artifact to maintain the British Empire as a world power for another century. She and William attack their captors while Vincent tries to launch the rocket. Helena goes to the elevation wheel and aims it further upward. When Vincent tries to stop him, she knocks him out and completes the realignment. The Trumpet activates, disintegrating Vincent and launching the rocket and itself up into space.
In the present, Pete proves skeptical of her claim. She admits that she thought it would be lost in space but Artie confirms it came down in Greenbury in 1962.
In 1962, Peggy and Billy are kissing and decide to go to the barn and see the site where Doug supposedly vanished. They approach the barn and the Trumpet activates, disintegrating a nearby cow. The two teenagers run off in terror.
In the present, a man hooks the Trumpet up to a radio telescope and aims it at the sky.
At the warehouse, Artie confirms that Jack Secord and Rebecca St. Clair saw the Trumpet in 1962 and reported it destroyed. He sends Jack and Myka to Ohio and Artie insists that they take Helena along with them.
In 1962, Jack and Rebecca come to see Bill and Peg at his mother Rose’s home. Nora Varley and her son Daniel come by. Rebecca talks to Daniel, who is wearing a toy space helmet and talks about how he’s a fan of the astronauts. He asks to help them fight the spacemen who took his father and his mother tells him to wait outside. John assures Nora that aliens aren’t involved.
The agents drive to the barn and Jack steals a kiss from Rebecca. Rebecca wonders if they should try a relationship given what usually happens to Warehouse agents. They spot the Trumpet and try to approach it, and realize that a control device has been attached to it with wax. When Jack tries to pick it up, it blasts out of control. They hide behind their car and the Trumpet disintegrates several objects, including part of their car.
In the present, Pete and Myka arrive at the barn in Greenbury and find the car from 1962. Pete finds Jack’s hat and an old science fiction fantasy magazine in the trunk. According to the report, Jack and Rebecca never found anything.
In 1962, Jack tries to separate the Trumpet from the control device by shooting it with a Tesla. When that fails, Rebecca shoots it with bullets. There’s a large explosion and when they check, they discover that the Trumpet is gone.
In the present, Pete finds the fittings and has Helena confirm that it’s from her rocket. They call Claudia who warns that the Trumpet isn’t the same and the thief is enhancing its power. A helicopter was destroyed a mile in the air. Pete snaps at Helena, telling her that she’s responsible for the deaths. Myka shuts off the hologram projector and reminds Pete that she came back, but he says that it doesn’t make a difference. They finally apologize to each other and then try to work out how the Trumpet got from Ohio to Pittsburgh. Pete looks at the magazine again and realizes that it has a kid’s handwriting on it. They figure that Daniel hid in the trunk picked up the Trumpet, and took it with him to Pittsburgh. Pete calls Claudia and tells her to find out everything she can on Daniel Varley.
In Pittsburgh, an adult Daniel prepares to focus more power through the Trumpet and send a transmission to GL-581 to make “them” hear him.
Pete and Myka go to Pittsburgh and he gets cheese-covered fries while they check in with Artie and Claudia. Claudia explains that Daniel was academically brilliant but was in and out of psych hospitals and became homeless and vanished off the grid. His last job was with SETI and he also taught under Professor Wilmore at PA Tech. The agents go there and Wilmore shows them Daniel’s old office. There are star charts for GL-581, a star that Wilmore explains is only 20 light years away and can support life.
The agents figure that Daniel believes that aliens from Gl-581 abducted his father and he plans to send a signal to them. Claudia confirms that all of the blasts came from the same location, the old SETI facility. Each blast was aimed at GL-581... and the next blast will hit Allegheny Field, killing 40,000 people.
Myka and Pete go to the SETI facility, closed due to cutbacks, and find Daniel. He has created a force field to keep them out and mistakes them for Jack and Rebecca. He refuses to listen when they warn that the next blast will hit the stadium.
Claudia confirms that there are 40,000 people at the stadium and Artie admits that he can’t use his government contacts to get the people evacuated in time.
Daniel insists that aliens abducted his father and Pete tells Myka to turn on the sphere. Helena appears and Pete tells her to explain to Daniel what happens. She tells Daniel that she built the rocket and he tries to shoot her to get revenge for his father’s death. Helena admits that Doug’s death was her fault but tells him that they all lose loved ones and the lost drove her insane with rage and pain. She tells him that killing thousands won’t make him feel better or bring his father back. Sobbing, Daniel asks why she did it and Helena says that it was an accident and she failed to keep the Trumpet from harming others. Daniel turns off the radio telescope with seconds to spare.
Back at the warehouse, Artie congratulates Claudia on her efforts. When she admits that it was nice to have Helena back, Artie tells her not to get used to it.
In 1893, Helena admits that she was eager to get into the scientist and depart for the stars. William how she reconciles building dangerous inventions on one hand and recovering artifacts on the other. She says that she looks to the future and expects great things, and says that the future will be a wondrous place.
In 1962, Jack and Rebecca look at Helena, trapped in the bronze sector. Rebecca notes that most warehouse agents up insane, villainous, or dead. However, she thinks that the two of them should enjoy their time together because they don’t know what the future will hold.
In the present, Myka puts away the Trumpet and Helena congratulates her for closing the case after a century. They both admit that they made a good team, and Myka wishes that Helena would have realized what she had sooner. Pete comes up and thanks Helena for her help. They shut down the seal and Pete puts on Jack’s old hat.