Joshua’s Horn is an artifact that can reduce anything to dust when it’s blown.
Myka brings H.G. Wells back in holographic form in order to find out as much about it as possible. In a flashback, we find out H.G. was building a rocket and one of her fellow agents thought the horn would be a good way to power it.
That sends the ship up and it crashes back down in 1962 where Warehouse agents Jack and Rebecca investigate it. The son of an astronaut steals it so he can try to contact his missing father in outer space.
Back in the present the son is talked down from using it and killing many innocent people.