Too Good to Be True

Richard Harland and his young son Danny arrive for a summer stay at their attorney Glen's home. They are greeted by Glen's guest Ellen Berent, who immediately attracts Richard's attention. Ellen's mother Margaret and sister Ruth are also staying there, setting up the web of relationships that drives the plot.

Ellen ends her engagement to Russell Quinton and seeks out Richard's arm. Glen tries to discourage the match, but Richard ignores him and moves forward with marriage. The rapid union signals the start of a dangerous obsession.

After the wedding, Ellen reveals a dark, controlling tendency. She manipulates the household, sows suspicion about the longtime caretaker, and pressures Richard by firing Danny's caregiver. The couple's bliss becomes fragile as Ellen's jealousy grows.

When Margaret and Ruth visit, Richard suggests using Ruth's artwork for the cover of his forthcoming book. Ellen takes a fit, exposing her insecurity and possessiveness. Her outburst underscores how she equates affection with possession.

Ellen has been teaching Danny to swim to the buoys while she sits in a rowboat. Danny cries out with a cramp; Richard senses danger and rushes to help by taking his speedboat out onto the lake. Ellen pretends to assist in the search, but Danny drowns before they can reach him.

After Danny's death, Richard withdraws, spending nights in a guest bedroom as he pulls away from Ellen. Ellen, sensing her hold slipping, initiates a return of intimacy and quickly announces she is pregnant. The period marks a fragile, uneasy happiness built on a dangerous lie.

Ellen confides to Ruth that she hates being pregnant and believes Richard only cares about the baby. She seeks an abortion, but her physician friend refuses to perform it secretly, forcing her to consider going elsewhere. The future looks precarious as Ellen's plan to control the marriage hangs in the balance.

Ellen sneaks back to the lake house and stages an 'accidental' fall, losing the baby. Richard muses that the miscarriage might be for the best and prepares to leave her for Ruth. Ellen, enraged by the threat to her control, begins plotting revenge.

Ellen learns of Ruth's involvement as Richard grows closer to her sister, Ruth, after seeing the cover art. Richard tells Ruth they need time apart and that he will go to New York for a few days. Ellen intensifies her plans for revenge as she realizes she is losing control.

Ellen exploits Ruth's trust by asking for barbiturate sleeping pills. Ruth provides them and Ellen secretly mixes the powder into a sweetener. At Ellen's birthday party, Ellen confronts Richard privately, admitting she let Danny drown because he paid more attention to Danny than to her. Richard slaps her and storms out.

Margaret and Ruth come to console Ellen after the confrontation, while Ruth realizes the danger of Ellen's plan. Ellen's conspiratorial tainted sweetener is stirred into iced tea, which Ellen drinks. She dies from the sleeping-pill powder as the scene confirms the long-buried motive for plotting revenge; the death is initially ruled a suicide.

Margaret finds the swapped sweetener packet and powder residue in Ellen's jacket; testing confirms Ellen's staged murder attempt. The district attorney, Ellen's ex-fiancé Russell, presents evidence against Ruth, but the charges are eventually dropped. Richard and Ruth are left at the lake house as they begin to rebuild.

In the film's closing moments, Richard and Ruth are seen together at the lake house, suggesting a new bond between them as they cope with Ellen's death and the tragedy that unfolded. The community settles into a quiet aftermath, with the past trauma lingering behind them. The lake remains a backdrop to their fragile connection.