Pretty much what it says in the title, really. This would involve the continued and varied meetings of two Immortals throughout the centuries of time. 1345 AD, England. 1677 AD, France. 1832 AD, the New World. 2010 AD, modern North America.
This setting allows for many different encounters of all types, not necessarily always with a happy ending or even the promise of another meeting. Perhaps as a Lady beset by brigands at one time, then centuries later meeting as courtiers in society's highest echelons? All that and more is possible.
But remember, no matter what feelings may develop between them, they still lead very violent lives; and never forget that in the end, there can be only one.