Anyone who has ever loved an old dog will love the idea behind filmmaker Melissa Dowler’s “Adele and Everything After,” though they may be given a moment’s pause by the film’s seemingly sad implications. The movie follows a Boston woman named Marty who suffers from a rare heart condition, and who receives a new lease on life courtesy of a big-eyed black Labrador service dog named Adele.

But with Adele entering her golden years, Marty faces the daunting twin challenges of finding a new canine companion possessed of Adele’s singular abilities, and of letting go of the relationship that has defined and transformed her life for nearly nine years.

But just what does “letting go” mean? Dowler doesn’t want to give away too much, but she acknowledges most dog lovers will probably need a little reassurance on the front end. “There is what you’d call a happy ending,” she says. “But it is a very emotional journey. We warn people to bring Kleenexes, but to know that in the end it will be okay.”

A corporate filmmaker by trade, Dowler had been looking to venture into documentary filmmaking for some time. As fate would have it, her months-long quest for a suitable subject came to an end in her very own downtown Boston apartment building, when she was approached by the lady who lived next door.

“Marty told me she needed to do a fundraiser, and she wondered if I could help her make a video,” Dowler says. “And when she told me her story, what she had been through and her relationship with Adele, I knew this was much more than a fundraiser. This was the documentary I had been looking for.”

Dowler learned that Marty suffered from vasovagal neurocardiogenic syncope, a heart condition characterized by sudden precipitous drops in blood pressure, resulting in unexpected fainting spells. The untreatable condition made a wreck of Marty’s life, rendering her a virtual shut-in, and leading to more than 30 concussions by the time she reached adulthood.

It seems improbable on the face of it, the notion that a dog could be of any assistance to someone with Marty’s problem. But when she looked into the possibility of employing a service animal, she found the Canine Partners for Life non-profit organization, and a solution to her problem in Adele, a black lab who had been trained to anticipate seizures in epilepsy patients.

With little additional training, Adele was able to do the same thing with Marty’s fainting spells. The two became inseparable companions, with Adele learning to perform an astonishing variety of additional tasks, from paying the check at restaurants to hauling clothes baskets back and forth from the apartment laundry room.

“No one really knows how these dogs do what they do,” Dowler explains. “But as soon as Adele got with Marty, she started sensing things. Whenever it happened, she would sit on Marty’s feet, wanting her to sit down. And it completely changed Marty’s life. Suddenly, she could go out and start doing all the things she could never do before.”

But the focus of Dowler’s film is  the painful segue for both dog and owner, the point where the nine-year-old Labrador can no longer perform all of her duties with the vigor that had characterized her youth, and where Marty must let go and learn to place her trust in a new canine friend.

“One thing people may not understand is this is not a normal dog-and-owner relationship,” Dowler says. “These two were together 24-7. If Marty rode an airplane, Adele was there. When she took a shower, Adele was waiting just outside the shower. It was like losing a part of herself.

“She was letting go of a relationship that saved her life. And she was racked with questions. Will I find a new dog? Will the relationship be the same? What will it mean for Adele?”

“Adele and Everything After” debuted in April at the Cleveland Film Festival, its premier drawing SRO crowds to showings on two different screens. Gratified by the reception, Dowler says the successful premiere speaks to the film’s compelling emotional center. “I was able to build a trust with Marty over time, and because of that, she doesn’t hold anything back,” Dowler says. “You really see her struggle.

“My own favorite films are films that make me feel something. And when one of my films makes the audience laugh or cry or gasp, that’s the best reward I can receive as a filmmaker.”

“Adele and Everything After” will screen at Scruffy City Hall Thursday, July 27 at p.m.