Warning: Spoilers follow
In it, two apes (Cornelius and Dr. Zira, played by Roddy McDowall and Kim Hunter) from the post-apocalyptic future where intellegent Apes have taken over the earth, from the previous movies, travel back in time to the present day as the Earth is destroyed at the end of Beneath.
The apes at first become celebrities, but when it is realized that they come from a world which enslaved humanity, the apes are then hunted down and killed. Cornelius and Zira have a son before the end of the movie, however, who is a main character in the third and fourth sequels, Conquest of the Planet of the Apes and Battle for the Planet Of The Apes.