"Chimpanzees: An Unnatural History" is a program that will probably make many viewers cry.

But Allison Argo felt she had to stay as dry-eyed and clear-sighted as possible while making this documentary, which she also narrates.

It couldn't have been easy.

The documentary, which on Sunday night launches the 25th season of PBS' "Nature" (see local listings), explores the sad story of generations of captive chimps — our very genetically close relatives, with almost 99 percent of the same DNA as humans.

"I try not to tell people what they should feel or think in the film," the filmmaker said.

"As I was writing the narration I kept saying, 'Just the facts. No comment. Don't get emotional,' and again when I was reading it, because you don't need to. Let people decide what they want to decide. Just present the story, present the characters, which are the chimps," Argo said.

Gloria Grow doesn't have any intention of being objective. Her eyes often rimmed with tears earlier this year as she accompanied Argo to a series of news conferences and interviews to discuss the documentary.

Grow and her husband, veterinarian Dr. Richard Allan, run the Fauna Foundation, which has become a haven for abused animals, including chimps used in biological research. Even chimps that were once people's pets, or performed to audience laughter in circuses and commercials, can end up in research facilities. Once they get to about five or six years old and can no longer be handled safely, they are often dumped in medical laboratories or imprisoned in isolation.

Grow's nonprofit foundation, based near Montreal, is featured in the documentary.

So, too, is Dr. Carole Noon's Save the Chimps group, of which she is founder and director. The nonprofit central Florida organization works to create a safe and suitable habitat for chimpanzees, such as those used in numerous experiments by the United States Air Force, which in 1959 captured dozens of baby chimps in Africa. These naturally social animals, whose life span in the wild mirrors humans, have long been locked in separate cages, taken out only to be used in grueling, dangerous and painful research, which may or may not benefit mankind.

One of the chimps featured in the program is Lou, a 42-year-old veteran of the Air Force programs.

The documentary is about "the chimps having a voice finally," said Grow. "Allison Argo was able to speak on their behalf ... about the tragedy of their lives."

The sight of an aged chimp, a victim of years of confinement, trying to summon up the courage to walk free beneath the sky, is just one of the many devastatingly emotional moments in Argo's movie.

"I'm not a raving animal-rights person, but I do think there's a need for accountability," said Argo.

She understands, she said, there are other points of view than the animal-lovers' about the use of chimps in research. But the medical community she tried to have a dialogue with, she said, chose not to respond.

"I couldn't even get the NIH (National Institutes of Health) to grant us an interview," she said, adding that laboratories can't or won't supply detailed records.

Despite all the sadness in the film, Argo feels it can be viewed in a positive light.

"I think people who laugh at the chimps in the commercials just don't know (what happens to chimpanzees). The purpose of this film is to open the window so that people can ... see what's on the other side, the dark side, and what the consequences are."