Near the end of Norm Macdonald’s new book, the comedian’s purported ghostwriter steps forward and offers a confession: “In order to fill up Mr. Macdonald’s memoirs, I had to stray far from the prosaic facts that made up his life and I have veered into straight-out falsehoods.” This will surprise no one who spends even a few minutes with “Based on a True Story,” much of which is bunk.

Based on a True Story, by Norm Macdonald (S&G) (S&G/S&G)

Macdonald is admirably uninterested in producing a standard comic autobiography. For this former “Saturday Night Live” cast member and onetime sitcom star, facts are mere starting points, fodder for a string of shaggy-dog stories that depict him as a fool, a criminal and a terrible role model. His specialty is extracting humor from grim scenarios. He takes a sliver of truth and spins it into madcap fantasy. He can be entertaining and objectionable within a single paragraph.

Typical of his approach is a deadpan yarn about a “terminally ill” 9-year-old with “a simple wish.” Thinking the boy wants a backstage glimpse of SNL, Macdonald takes him to NBC’s studios. The child soon admits that he selected Macdonald as his tour guide not because he’s funny but because he’s Canadian. His real wish, the youth explains, is “to kill a baby seal.” Together, they travel north, where, in lurid fashion, the boy’s dream comes true. From there, things get darker.

Like many of Macdonald’s fictional anecdotes, this one transforms a believable framework into farce. In another tall tale, he’s convicted of trying to arrange the killing of an SNL writer, part of a scheme to steal the man’s girlfriend, then-cast-member Sarah Silverman. As Macdonald’s stories go, this one’s irredeemably lame — it involves a long, hackneyed joke about jailhouse rape and a 40-year prison sentence that’s inexplicably commuted after four months.

Although he doesn’t earnestly address his diminished post-SNL profile, Macdonald revisits his removal from the show’s “Weekend Update” segment, which reportedly occurred because he pilloried NBC executive Don Ohlmeyer’s friend O.J. Simpson. But don’t expect to learn what really happened. He impishly says that Ohlmeyer wanted more Simpson jokes, not fewer.

The droll put-ons extend to the book’s authorship. The aggrieved ghostwriter presented here is clearly fanciful. Still, some genuine emotion is on display. Recalling his childhood, Macdonald tells of slipping into a “five-year walking sleep” after a troubling, if ambiguous, encounter with an older family friend. Later, when hearing the man’s name, he says, “I couldn’t find my breath and I snatched my older brother’s sleeve in a panic.” He’s just as forthcoming about his “twenty-year gambling spree”: “It’s true I lost it all a few times.”

Macdonald is at his most disarming when describing the thrill he felt as fans first started to recognize him.

“They’d laugh and dance like they’d won a prize,” he writes, “and I’d just stand there and smile and feel warmth from their love.”

Kevin Canfield has written for Film Comment, Bookforum and other publications.

On Sept. 22 at 7 p.m., Norm Macdonald will be in conversation with Washington Post national arts reporter Geoff Edgers at Sixth & I Historic Synagogue. For tickets, click here.

Based on a True Story
A Memoir

By Norm Macdonald

Spiegel & Grau. 256 pp. $28