From the introduction:

Don’t be deceived by its simple appearance. What looks like a strangely placed door or free-standing oddity is a gateway that leads to a hyper-dimensional machine whose single entry can be moved to any point of history in the universe. Step across its threshold and enter the impossible. Enter the door to infinity.

The Earth you know has been a peaceful place, a sanctuary for humanity where they have blissfully been consumed by their own petty concerns, unaware of the great dangers all across time and space. Heroes, such as you, have used the Door to Infinity to safeguard it and a thousand other worlds just like it. Here, in my final moments I can offer you only this brass key and a few simple words, you are the sum of your memories and the equation cannot be changed. Once you step through the door you will remember all the possibilities you see even as time washes away the never weres and could have beens. Don’t contact yourself in the past or the future, even being near yourself risks the darkest of fates, it is forbidden. Find companions for your journey and treasure them dearly, infinity is a lonely place.”

– A dying old man in a Victorian coat.

Adventure any where, any time. It sounds like a recipe for endless vacation right? It would be if not for all the crazies that want to undo the universe from either end. Nope, having your own time machine comes with a lot of responsibility. The heroes were chosen by chance, fate or some higher order to step outside the ordinary flow of time, to see a wider vista than others were ever meant to see. They may do it reluctantly or without understanding their purpose in the beginning, but when they’re charged with shepherding the entire universe, eventually the will heed the call or everything might just cease to be. It’s their choice.

With the whole of creation before them, it could be difficult to decide what to do. Sometimes they will find clues that they will want to check out later, but often adventure finds them no matter where they are. That is just the nature of being a hero.