There is a moment in the documentary “Spirit Unforgettable” in which Spirit of the West frontman John Mann forgets the lyrics to the band’s signature anthem.

It’s not an innocent or momentary lapse, but a very poignant sign of his struggle with early onset Alzheimer’s disease. The audience at Toronto’s Massey Hall sings the iconic Home for a Rest in unison as a clearly moved Mann finds his place on the iPad in front of him.

The opening lyric of “You’ll have to excuse me, I’m not at my best,” had taken on an entirely new meaning.

Mann, 53, and his wife Jill Daum allowed filmmaker and close friend Pete McCormack to follow their journey for about six months, capturing the effects of the devastating diagnosis.

“Spirit Unforgettable,” which premiered at Toronto’s Hot Docs film festival this spring, will air on HBO Canada July 1.

“It’s a struggle. I know I am not the only kid on the block who has it,” Mann says in the film, wiping tears.

“It's like a white wall, an empty white wall … Nothing else.”

Mann was known as a flamboyant, sometimes outrageous stage performer.

But two years ago he began to notice memory lapses that only grew more severe. At first he and his wife chalked his issues up to anxiety. Then came a colorectal cancer diagnosis.

But Mann’s mental decline continued until doctors near his home in Vancouver gave him the grim news.

There was an initial “grace,” said his wife, but the progression of early onset Alzheimer’s is relentless.

“The meds really are no longer able to kind of postpone the symptoms,” Daum told CTV News. “And the decline happens quite rapidly."

The documentary is a love story, the traditional girl-meets-boy kind, and the special connection of the Celtic-inspired rock band that recorded 13 albums together starting in 1984.

Daum says she fell for Mann immediately upon meeting him more than 30 years ago but Mann was in love with someone else at the time. So Daum waited, impatiently.

“He didn’t have a chance, I stalked him so mercilessly.”

But for Daum, the true narrative of the documentary is the “brotherhood” of the band.

“I find it stunningly beautiful to watch these men jump in to support John and to fill in the gaps. And what they do with grace and humour and then make this incredibly uplifting music and create fun for so many people.”

She is writing a play about their journey and the couple hopes their story can offer hope to others facing the same struggle.

“He is so valiant and he works so hard to wring out what he can from every day,” Daum said.

Doctors hope Mann’s celebrity will lead to more awareness of the disease.

“Alzheimer’s disease is a very common illness with almost 750,000 Canadians affected,” said Dr. Carmela Tartaglia, a neurologist at Toronto Western Hospital.

Daum yearns for more support for those with early onset dementia and those who care for them.

“Every day you watch the person you love disappear more, and every day you have to take on more responsibility.”

With a report from CTV Toronto’s Pauline Chan, CTV medical specialist Avis Favaro and producer Elizabeth St. Philip