Author Richard Harland Patrick Duffy and his son Danny Harland Neil Patrick Harris spend the summer with Richard’s longtime friend and attorney, Glen Robie Larry Drake, at Glen’s lakeside home. Richard is a widower whose wife died in a car accident that left Danny crippled, a tragedy that shadows their new arrangement and shapes every decision they make. When they arrive, a striking woman named Ellen Berent Loni Anderson catches Richard’s eye, and she’s there with her mother, Margaret Berent Julie Harris, and sister, Ruth Berent Glynnis O’Connor. Glen tries to steer Richard away, mentioning Ellen’s engagement to Russell Quinton James B. Sikking, but Richard’s charm is undeterred, and Ellen quickly becomes smitten in return, choosing to end her engagement to Russell to pursue Richard instead.
The romance accelerates into marriage, and Ellen’s façade of sweetness soon cracks to reveal a darker, more possessive side. At the lake house, she sows doubt and jealousy—taunting the longtime caretaker by insinuating he hit on her and firing Danny’s trusted caregiver. When Margaret and Ruth visit, Richard proposes using Ruth’s artwork for the cover of his upcoming book, a request that sends Ellen into a jealous tailspin. Ellen has been teaching Danny how to swim to buoys, while she sits in a rowboat, claiming to help as Danny cries out in distress. When Danny pleads for help, Ellen pretends to search with him, but the cries fade into the water, and Danny drowns.
The tragedy drives Richard inward, and he begins sleeping in a guest room, distancing himself from Ellen. Desperate to maintain control as his affection wanes, Ellen sneaks into the room, and they consummate what she hopes will be a lasting bond. Ellen reveals she is pregnant, and for a while the couple seems to drift into a fragile happiness. Yet Ellen’s fear and insecurity resurface as she confides to Ruth that she hates being pregnant and feels Richard cares more about the baby than about her. When Ellen learns her abortion won’t be easily arranged—her doctor friend can’t perform it without risking exposure—she looks for a way to ensure anonymity and control.
Ellen eventually schemes to regain power by staging a personal crisis at the lake house. She slips away to the lakefront, where she stages an “accidental” fall and loses the baby. Richard, increasingly distant from Ellen, reveals to Ruth that he plans to leave and return to New York for a few days, hoping for space. Ellen’s anger boils into a plan for revenge, and she exploits Ruth’s trust to obtain sleeping pills by confessing to sleepless nights and nightmares. Ruth, hoping to help, goes to fetch the pills, but Ellen secretly replaces the contents of the capsules with talcum powder and swaps the bottle back.
At Ellen’s birthday party, she and Richard share a private talk that turns into a confrontation about Danny’s death. Ellen admits to letting Danny drown because Richard paid more attention to him than to her. Richard lashes out, striking Ellen and walking away in anger. Ruth and Margaret come to comfort her, and Ruth, unaware of Ellen’s ruse, stirs a sweetener packet that Ellen had previously prepared with the powder substitute into iced tea. Ellen drinks the tainted tea and dies, a tragedy initially ruled a suicide.
The case soon shifts as the district attorney, Russell Quinton, presents evidence that Ruth was jealous of Ellen and had motive to harm her. Margaret discovers the altered sweetener packet and powder residue in the jacket Ellen wore at the party, and testing confirms Ellen’s attempt to stage her own murder. With the truth exposed, the charges against Ruth are dropped. In the end, Richard and Ruth are seen back at the lake house, facing an unsettled future but connected by the events that bound them together.
