Ellen Jones, a devoted housewife, lives in a suburban neighborhood with her husband, George, who is bedridden due to heart disease. Their seemingly tranquil life takes a turn when George becomes mentally unstable, suspecting Ellen of plotting against him with his doctor and former college friend, Dr. Ranney Grahame. George accuses the duo of trying to kill him for financial gain. In a fit of paranoia, he writes a letter denouncing their alleged conspiracy and gives it to Ellen to mail, threatening her not to read it. Ellen sends off the letter after being coerced by George, but panic sets in when she realizes the severity of its contents and its potential consequences.

As George's paranoia spirals, Ellen seeks to retrieve the letter before it falls into the wrong hands. She navigates a tense situation, dealing with suspicious neighbors and post office bureaucracy while racing against time. Despite her efforts to maintain composure, Ellen feels the pressure mounting as George's condition worsens. The film weaves a gripping narrative, capturing the claustrophobia and tension as Ellen battles to prove her innocence against the backdrop of an unraveling domestic life. The atmosphere of uncertainty and fear permeates every interaction she has, highlighting the fragility of trust within close relationships. Ellen's desperate pursuit unfolds in an escalating confrontation with both external elements and her own inner turmoil.