"This is the most fascinating tape on the entire Earth," says Joseph "Run" Simmons. What's he watching? The Zapruder tape? Citizen Kane? Aretha Franklin's Amazing Grace?

Nope, he's watching the unedited footage MTV shot during the 1986 recording of "Walk This Way," Run-DMC's remake of Aerosmith's 1975 rap-adjacent rock song. Although it became one of the defining singles of the '80s, the track's recording was hardly auspicious. Everybody involved knew they were essentially making a gimmick, a play to break Run-DMC into the mainstream and to juice Aerosmith's sagging comeback attempt.

Guitarist Joe Perry wasn't thrilled that producer Rick Rubin wanted him to recreate, not reinvent, his original part — right down to the solo. Run and DMC had been freestyling over the song's beat for a decade, and didn't see why they suddenly had to start using Steven Tyler's bawdy lyrics. Tyler (assisted by a mountain of cocaine) was game, writing the lyrics down for the rappers but not caring too much about how they came out when the duo hit the mic.

The most crucial musical catalyst, as throughout Run-DMC's career, was DJ Jam Master Jay, who showed Tyler and Perry how hip-hop DJs would loop just the intro of the song, before the guitars came in. DMC and Run were among the rappers who didn't even know the song's name: they'd just call for "Toys in the Attic, track four." It was also Jay who got his bandmates back in the studio to recut their verses, after Tyler and Perry had left. He knew they were leaving money on the table if they didn't end up with a usable track.

Geoff Edgers's new book Walk This Way is subtitled Run-DMC, Aerosmith, and the Song That Changed American Music Forever. He strenuously makes the case that the track merits a book-length examination, but he doesn't really have to: Run himself seems to agree, and most of the book isn't even about the actual song.

The first half of Walk This Way devotes alternating chapters to the parallel histories of the two groups involved. A single long, coherent chapter on each would have been more readable, but Edgers makes his point that the two groups were both inhabiting different sections of a shifting musical landscape.

Tyler's a talented drummer, and Edgers spends pages hashing the debate over who invented the "Walk This Way" beat, Tyler or Aerosmith drummer Joey Kramer. (The answer seems to be that Tyler pounded out the essential rhythm, and Kramer refined it.) The upshot is that Aerosmith were always a more rhythmically ambitious unit than their peers, and songs like "Sweet Emotion" combine singing with proto-rapping. After Perry left the band for the usual '70s rock reasons (drugs, creative tension, and more drugs), he came back in the early '80s for a reunion that proved sobriety in and of itself wouldn't rekindle any magic.

Tracing the history of Run-DMC, Edgers argues that hip-hop artists were persistently frustrated by the fact that before "Walk This Way," music programmers didn't really know what to do with the genre. "Rapper's Delight" and "The Message" got some exposure, but felt like novelty hits to a mainstream audience; black audiences appreciated the music's urgency, but the African-American radio establishment still preferred the smooth sounds of Shalamar and Peabo Bryson.

In Edgers's telling, artists like the Sugar Hill Gang were too good in the studio: their slick productions didn't demonstrate rap's raw force. He doesn't go so far as to say Run-DMC wouldn't have broken out without "Walk This Way" or even the Raising Hell album generally, but the fact is, that album and single did happen, catalyzed by Rick Rubin and Russell Simmons of Def Jam Recordings.

Edgers describes Rubin, a shaggy dog of a genius who founded Def Jam in his NYU dorm room. He loved both rock and hip-hop, and intuited a path to mainstream success that combined elements of both genres. He would be the eccentric with the golden ear, while Simmons (Run's older brother) would build the label into a hip-hop colossus.

Rubin knew Raising Hell could be a blockbuster, but he also knew MTV and Top 40 radio would need a single that could wedge Run-DMC onto their playlists...and knew a cover of "Walk This Way," an almost singularly hip-hop-friendly classic rock track, was the way to do it. Reporters from MTV and Spin helped suggest the collaboration, and were on hand to report during the session.

The shoot for the music video spanned two long days. Director Jon Small put the two bands in adjacent rooms, then had Tyler symbolically break down the wall between the two. The rest of Aerosmith were played by another band (leaving Aerosmith's actual band members to later muse that they should have just flown themselves up to New York for the shoot), and a member of that band remembers how odd it was that Tyler talked to him more than to any member of Run-DMC.

Some of Edgers's most amusing anecdotes regard the video shoot, from Tyler's advice to the rappers on how to rouse yourself for morning music duties (it involves a sexual act) to the singer's frustration at how well-constructed the prop wall was ("It had to have a little tension in it," says Small about Tyler's athletic attempt to bust through) to the fact that Run-DMC's largely black audience was conspicuously more excited for the concert video shoot than were Aerosmith's indifferent white fans.

Although the two groups would tour together in the 21st century, they never became BFFs. Art brought them briefly together...and if it was a gimmick, it totally worked. (The first station to break the single, Edgers notes, was WBCN — Aerosmith's hometown station, desperate for something half-decent to play.)

After "Walk This Way," hip-hop was a chart force, and Aerosmith were on track for a late-career revival that in some measures eclipsed even their first run of success.

It couldn't save Run-DMC, though. While the band stayed together, more or less, for years, they never reclaimed the artistic and commercial peak they hit with Raising Hell. Ironically, Darryl "DMC" McDaniels tells Edgers, he wanted to go more rap-rock than his bandmates did.

"Me," he says, "I wanted to become a Rage. I wanted to become a Cypress Hill. But they kept telling me, 'Nah, D, you just do what you was doing in '86.'"

If it ain't broke...