The Beast Within was the 19th episode in Season 5 of Quantum Leap, also the 94th overall series episode. Written by John D'Aquino, the episode, which was directed by Gus Trikonis, premiered on NBC-TV on March 16, 1993.
Sam is Henry Adams (played by Mike Jolly), a Vietnam War veteran living in the forest with an epileptic fellow veteran who will die unless he gets his medicine. However, he is opposed in this task by the sheriff living in town, who served in the same unit and doesn't want to face his past.
Leaping in to find himself sitting on a windowsill outside a dark room at night, wearing a furry long coat and holding a bottle of pills in his hand, Sam is startled into falling off the ledge by a teenage kid who mistakes him for Bigfoot. Stumbling away into the forest to avoid being captured by the house's owner who comes after him, Sam soon runs into a scrawny, wild-haired young man who is revealed to be his host's friend, coming to meet Sam - as Henry Adams (leapee played by Mike Jolly), as he learns his host is named – on the way home. Home, as it turns out, is a secluded campsite in the forested mountains, away from the town where Sam found himself when he leaped in.
When Al finally arrives to give him some answers, Sam learns that Henry is a Vietnam vet living with his friend Roy Brown (Sean Sullivan) alone in the mountains of Washington State. Roy and Henry went to Vietnam, a few years back, along with their two best friends from high school, John Burke (John Hilliar) and Luke Marlet (Pat Skipper), but an incident on the battlefield, in which a moment of moral disagreement led to John's sudden death, caused Luke to end his friendship with Henry and Roy, who were the only witnesses of that tragic death and its cause, and who have nevertheless remained silent about it.
In Vietnam, Roy was a "tunnel rat", a burrower who is assigned the duty to flush enemies out of tunnels and cracks. During one later mission he was shot in the head and has since been suffering from nervous spasmodic fits. Coming back home to live with Henry in the wild, Roy has been supplied the medicine he needs for free by the town doctor. However, the man has very recently died, leaving Roy doomed to eventually die of his nervous disease, as there is no other way of getting the pills he needs – Luke is now the town's sheriff, and threatens Roy and Henry with arrest on the charge of a petty theft Roy has recently committed, if they even show their faces around town. The only way to get the pills was to have Henry rob the local pharmacy, at which point Sam has leaped into the scene.
However, now that Roy asks Sam for the pills he was supposed to bring, Sam discovers he had dropped the bottle after having been surprised by the boy on the windowsill. Sam is now forced to come up with a way to delay Roy's worsening seizures by using the means available in nature; He must go into town and try to persuade Luke not to turn his back on his two old friends, and also not to arrest Sam himself on the spot; and finally, he must save the life of Daniel Burke (David Tom), the boy who had startled him upon his arrival – who is none other than the teenage son of the late John, now raised by Luke, who had married John's wife after returning from the war. Having once been shown a glimpse of the mythical Bigfoot by his late father (and having recently mistaken Sam in his furry coat for the monster), Daniel is angry with his stepfather Luke for refusing to believe in the creature's existence, and runs away from home to get his own photographic proof of it. But before he gets the chance, his life may be in danger in the wild terrain of the mountains.