An anonymous caller invites a number of guests to a party, telling each one that the party is in his honor. The guests include Dr. Murray Reid, a university professor, Henry Abbott, a campus radical, Jason Osgood, the city's mob boss, Sylvia Inglesby, an attorney, Timothy Cronin, Sylvia's district attorney, Margaret Chisholm, a society matron, James Daley, an author, and Jean Trent, Henry's love interest. When the guests begin to arrive, they find that each is an enemy of another guest. As they have been instructed by the host, the butler and his assistant turn on the radio and a voice announces that the guests have been gathered for a game of wit in which death will be the "ninth guest." Later, the guests discover a dead body in a closet, and at eleven o'clock, Osgood dies trying to poison the others. Then, Jean tells the others that Margaret received a letter revealing that she is a bigamist and had committed her husband to an asylum. Distraught at having been found out, Margaret kills herself at midnight. Soon, the remaining guests are bickering among themselves, and when they find a gun on Timothy, Sylvia, attempting to defend him, kills him. Sylvia then commits suicide by throwing herself against the electric gate. At two o'clock, the lights go out, and Dr. Reid is shot and Henry is wounded. As James ties Henry up, sure that he is the killer, he explains that the first body was that of the electrician who wired the apartment for Henry, and that the radio voice was activated by a button located on Henry's chair. Finally, Henry admits his crimes and explains that Margaret's husband was his brother, and that Sylvia was Margaret's attorney and Timothy was a corrupt district attorney. Aware that his love for Jean is hopeless, Henry lets her and James leave, then electrocutes himself.