Jack Dalton, a lifeguard, watches in awe as Mia Simone, a vision of loveliness, looms on the dune above and wends her way down to the beach below. When she enters the water and disappears, he leaps off his chair and plunges in after her. Upon carrying her back to shore, he performs mouth-to-mouth resuscitation and saves her life.
Because he thinks she was trying to kill herself, he visits her at the hospital and confronts her about it. Indignant at his innuendo, she scolds him; as a peace offering, he hands her a bouquet of flowers and, then, discreetly leaves.
A week later while kayaking on Scargo Lake, he passes a dock and sees her dozing on a chaise lounge. So astonished is he at the sight of her that he drops his paddle. When he dives in after it, he surfaces by the dock and finds her looking down at him. While rowing him out to fetch his kayak and paddle, he apologizes and invites her out to dinner. To his surprise, she accepts his offer.
Both enjoy the dinner date and, so, begin seeing one another. Jack, who has fallen in love with her, makes his feelings known. She, however, insists they can never be more than friends. Secretive, she, also, refuses to tell him anything about herself.
Only when a hit man forces his way into her home and Jack restrains him, does he learn that she is hiding from members of the Boston mafia. Not content with having murdered her boyfriend and father, they are now hell-bent on killing her to keep her from testifying against them. Too, she confesses to Jack that she was, indeed, trying to take her own life that afternoon he for the first time had seen her from his perch on the lifeguard's chair.
The rest of the story depicts Jack's attempts to get Mia out of harm's way and into the witness protection program of the FBI.