In Washington, D.C., police captain Frank Matthews’s career is on the rise, having just been appointed consultant for a powerful U.S. senator. His domestic life, however, is questionable. He suspects his wife of having an affair with an old flame. One evening, after appearing at a political function in Baltimore, Matthews decides not to return home until the following morning. The next day, he is informed by authorities that his wife has been discovered shot to death while in bed with her lover, who was also killed. Soon, Matthews is made aware that his own colleagues, the police, have made him the prime suspect in the case.

Pendulum also features a side-plot involving a death-row inmate, Paul Sanderson, convicted of rape and murder, who is set free due to a legal technicality. Sanderson had been originally tracked down and arrested by Matthews, who views these circumstances as a grave injustice. Ironically, now that Captain Matthews is a suspected murderer, he hires Sanderson’s lawyer, Woodrow Wilson King, to represent him. For the remainder of the feature, these two storylines intersect until the film reaches its violent conclusion.