Nearly every veteran rock, pop and country star of the last half-century has launched a “Farewell Tour” — which is usually followed by a half-dozen more Farewell Tours. The Who have been saying goodbye for a quarter-century.

When the country music legend Glen Campbell embarked on his “Goodbye Tour” in 2012, there was never any doubt these were to be Mr. Campbell’s final live shows. The real question was whether Campbell would be able to complete the tour, as he had been diagnosed with Alzheimer’s disease, and the effects were rapidly escalating from insidious to heartbreakingly obvious.

Directed by the veteran actor James Keach, who had nearly unlimited access to Campbell, his wife and three of his children, “Glen Campbell: I’ll Be Me” is a poignant, stark, lovely and sometimes devastating film — a tribute to one of the great crossover stars of his time, and an unblinking look at how Alzheimer’s relentlessly chips away at one’s memories and thought process, brick by brick. It is worthy of an Academy Award nomination.

In the late 1960s and early 1970s, Glen Campbell was one of the biggest entertainers in the world. With his unmistakable voice, Arkansas choirboy good looks (though Campbell was anything but a choirboy), smooth vocals and virtuoso guitar skills, Campbell was a star onstage, on TV, on the radio and even in movies. He scored a number of Top Ten hits with songs such as “Wichita Lineman,” “Galveston” and “Rhinestone Cowboy,” he was the host of “The Glen Campbell Goodtime Hour” on TV — and he even starred alongside John Wayne in “True Grit.”

Related:

James Keach talks about directing “Glen Campbell: I’ll Be Me”

Review of Glen Campbell’s 2012 concert in Joliet

Glen Campbell in 2012: ‘I’ll be very pleased with whatever my legacy is.’

The film races through a montage establishing Campbell’s credentials for generations too young to remember when he was a superstar (Campbell is now 78). We also see Campbell and his fourth wife, Kim, watching old home movies with some of his children. At times Kim has to remind Glen of the names of his kids.

At the Mayo Clinic, a doctor asks Campbell what year it is, what the season is. A frustrated Campbell blurts, “I don’t need to know all that.” His wife talks about Glen not being able to find the bathroom in his own home. Rehearsing with a superb group of studio musicians known as the Wrecking Crew (along with children Cal, Shannon and Ashley), Campbell grows frustrated when he can’t remember lyrics or guitar licks. Day by day, he’s slipping away.

And yet when Campbell takes to the stage on that Goodbye Tour, with his band and the audience collectively holding their breath and hoping he’ll get through it, the result is often transformative. Yes, Campbell occasionally struggles with the lyrics, even with the help of a teleprompter, and sometimes he’ll play a song twice in the same show, but he still has that magnificent and instantly recognizable voice, and at times his fingers are flying so fast on the guitar it’s a blur. A blazing duet of “Dueling Banjos” with daughter Ashley is pure, breathtaking magic.

Campbell’s tumultuous personal life is glossed over in “I’ll Be Me.” He’s been married four times and he has eight children, and he was a tabloid fixture back in the day. “I’ll Be Me” is about Campbell’s life now. Kim has been with him for 32 years, and, like so many who have a spouse or partner with Alzheimer’s, she makes incredible sacrifices and pretty much puts her own life on hold to tend to Glen’s every need. Their children, all terrific musicians, are clearly treasuring every moment they get to play onstage with their father while he still knows their faces.

One of the most heart-wrenching scenes in the film: The Campbells are in Washington to do a show and to make an appeal on Capitol Hill for more funding for Alzheimer’s research. Ashley delivers a moving speech about her father’s condition while Glen sits near her. It’s impossible to tell how much he understands about what’s happening in that very moment.

In addition to the footage of Campbell at home, in doctor’s offices and onstage, “I’ll Be Me” features testimonials from Bruce Springsteen, the Edge, Brad Paisley, Jimmy Webb (who wrote many of Campbell’s biggest hits), Blake Shelton and a number of other stars who speak eloquently about Campbell’s talent and influence. Keach knows the story he’s telling is at times going to be tough to sit through, and he finds just the right pacing and tone.

One of Glen Campbell’s final songs was “Not Gonna Miss You,” dedicated to his wife. I can’t imagine anyone listening to it and not being moved.

[s3r star=3.5/4]

Area23a presents a documentary directed by James Keach. Running time: 104 minutes. Rated PG (for thematic elements and brief language). Opens Friday at AMC River East 21 and AMC Northbrook Court 14.