A 25th-century hero got some 21st-century help to reverse his obesity and save his life.

Gil Gerard, best-known as the handsome star of the well-remembered sci-fi series “Buck Rogers in the 25th Century,” had ballooned to more than 350 pounds (leading to diabetes, heart disease and other life-threatening conditions) before deciding to undergo drastic surgery in October 2005.

Cameras followed him before, during and after the surgery, enabling him to tell his own story in a shocking new special premiering tonight at 10 on Discovery Health Channel (Ch. 115 on Time Warner Cable in New York).

In the show, you’ll see how Gerard, 63 – who weighed 175 pounds in his TV heyday – had packed on so many pounds that fans didn’t recognize him during personal appearances at sci-fi conventions and collector confabs.

And, since this show is on Discovery Health – a cable channel known for its unflinching depictions of medical procedures – you’ll go inside the operating room at a Henderson, Nev., hospital to witness Gerard’s amazing 30-minute surgery, an operation known as an “MGB” or mini-gastric bypass.

Among those on hand to lend support are “Buck Rogers” co-star Erin Gray, 57, who has a tearful reunion with Gerard nearly a year after the surgery, by which time he had shed 140 pounds.

Looking back, Gerard asks, “If I hadn’t had this surgery, where would I be?

“If I was lucky,” he answers, “I’d be dead because I wouldn’t want to be living the life I would have been living a year later if I hadn’t had the surgery. I really did not want to die.”