Natasha has been on her own for as long as she cares to remember, which isn't very long at all. She's had her heart broken before and isn't too keen to repeat the experience. Terrified of getting hurt, she closes herself off, pursuing relationships only if she's certain that there's no future in it.
Travis isn't allowed to waltz into her life and pretend nothing's changed between them, but he does exactly that. He's crushed her before and she isn't going to let him do it again... but there's something about his magnetic gaze that renders her completely weak-kneed and helpless to resists.
When Natasha falls, she falls hard. She doesn't know if she will be able to climb back out, this time.