Once upon a time travel past came through the future.
Yes, Dr. Gilgamesh. You are all that stands in the way of these reptilian shape-shifters and the portals of time. Should you choose to slip into the vortex of time and challenge these surly creatures, you will have to be quick.
The shape-shifters of the planet Tirmat are taking full advantage of the break in the time continuum. And they are perilously close to establishing a base in our past. If allowed to pass through the time portals, even a small Tirmatian force would overwhelm the armies of Earth's past.
Go quickly, Dr. Gilgamesh. The Tirmatians are nearing the time portals of Medieval Times, Ancient Rome, Ancient Egypt and the year 2097. Earth history, this day and your very existence may never come to pass!