Dr Hasslein was a physicist and special scientific advisor to The President. In the first and second movies, the space flight project sent astronauts Taylor, Dodge, Landon, Stewart, Maddox and Brent to the world of the apes through something called a Hasslein Time Curve – a form of time dilation possible with their spacecraft. It's assumed that this was named after the Dr. Hasslein who we finally see in the third movie.
Hasslein learned that the talking chimpanzee 'ape-o-nauts', Doctors Cornelius, Zira and Milo (who was killed by a primitive gorilla shortly after arriving), had actually arrived in the present day (1973 in the movie) from Earth's own future, where mankind had fallen to the level of beasts while apes rose to power and intellect. While the other members of the Presidential Commission appointed to deal with "alien visitors" were initially skeptical of the time-travel story, Hasslein believed them, seeing it as confirmation of his theories. He privately became very suspicious of what their arrival could mean for mankind; afraid that the pair's presence might somehow set man's downfall in motion. He explained to a television news presenter his theories of time, and his belief that changing the future may be possible. He analogized time to be a highway with an infinite number of lanes, all going from the past to the future; by changing lanes, one can change destiny (see 'Trivia').
After gaining their trust, the apes revealed to him Earth's eventual fate. As the President's science advisor, he expressed his concerns, resulting in Cornelius and Zira first being taken into custody, then interrogated at length. Provoked by Zira's drugged admissions of her experimentation on humans, and Cornelius giving details of his historical research into man's decline, the Presidential Commission concluded (in a reflection of Taylor's fate before the ape tribunal) that the couple's unborn foetus should be "prevented" from birth, and that Zira and Cornelius should be "humanely rendered incapable" of conceiving again, with their ultimate fate to be decided later – at Hasslein's determination - though it appeared likely they would be handed over either to scientists or the military for study.
When Cornelius and Zira escaped military custody (at the beginnings of Zira's labor pains), Hasslein lobbied the President to kill the apes before they could threaten man's existence, and mounted a full-scale hunt, including searches of all local circuses and zoos. Days later, a carpet bag abandoned by Zira turned up near the derelict shipyard where they had been hiding, and the pair were spotted soon afterward with Zira carrying a baby chimp - Hasslein now knew she had given birth. Boarding their ship, Hasslein approached Zira, pistol in hand, and demanded she give him the baby, as the authorities approached. Before they arrived, he shot Zira, and fired several shots into the swaddling blankets. Cornelius, who had earlier asked Lewis Dixon for a pistol "to kill ourselves" and avoid being captured, now used the pistol to avenge his wife, shooting at Hasslein from a crow's nest on the ship. As the authorities arrived, Hasslein and Cornelius traded gunfire. Cornelius killed Hasslein, but was himself killed by sharpshooters, falling to the deck. Despite Hasslein's best intentions, the baby he killed was not the offspring of Zira and Cornelius – and his treatment of them may have just set in motion everything Hasslein hoped to prevent.