Amnesia

Amnesia.

is an abnormal degree of forgetfulness and/or inability to recall past events. Depending on the cause, memory loss may have either a sudden or gradual onset, and memory loss may be permanent or temporary.

Cassie was just a girl who always carefree, but always had horrible things happening to her.

Niall was a guy who watched her her fall and get back again, getting stunned by seeing it happen.

What happens when Cassie falls too hard and Niall is the only one to help her get back up, but of course he's not aloud to help the only person who is in need.