A "black star" slingshots Enterprise back in time, to the 1960s, in Earth orbit. She appears almost directly above a military base, which draws all sorts of unwelcome attention. The base dispatches Major John Christopher to look. Enterprise attempts to keep him at bay with the tractor beam, but his aircraft is too fragile to withstand the beam and disintegrates, forcing Enterprise to beam Christopher aboard. Now they have disrupted history. Christopher himself is not historically significant, but his son is. He must be returned, but how?