The fourth Planet of the Apes film is set in 1991, 20 years since the assassination of talking, time-traveling apes Cornelius and Zira at the end of Escape From the Planet of the Apes. The couple's infant son, Caesar (Roddy McDowall), has grown to adulthood in the care of kindly circus owner Armando (Ricardo Montalban).
Meanwhile, a plague has wiped all dogs and cats from the face of the Earth; speechless primitive apes have therefore been domesticated and turned into first pets, then servants of humankind.
Caesar becomes outraged at the treatment of these simian slaves and accidentally reveals his powers of speech in front of the militaristic authorities, who kill Armando when he tries to protect his friend's identity. His cover blown, Caesar kick-starts a revolution that pits chimps against humans, paving the way for eventual ape ascendency.
Caesar was the second of McDowall's three Planet of the Apes characters; he also portrayed Cornelius in the first and third films and Galen in the short-lived 1974 television series.
After taking over the franchise with this picture, Hollywood veteran J. Lee Thompson would become the only director to helm two Planet of the Apes films when he returned for the fifth and final installment.