The band Traffic has probably been getting its heaviest web traffic ever this past week, thanks to the use of a 50-year-old song, “Dear Mr. Fantasy,” that plays briefly but inescapably in the first 10 minutes of the superhero epic. It’s the most prominent of the handful of pop songs that show up during the three-hour running time, at least before an even older oldie, from 1945, brings up the end credits.

And boomers who are well familiar with the Traffic tune as an FM staple for the last half-century may not be that much less initially puzzled by its appearance than millennials: Why isSteve Winwood warbling aboard the starship Benatar?

(Warning: Our answers include mild thematic spoilers, for the hundred or so arthouse partisans and shut-ins who haven’t seen the film.)

Of all the movies you might walk out of saying, “Man, music supervisor Dave Jordan really earned his salary on that one,” “Endgame” does not rank high. Like other “Avengers” movies, it doesn’t interrupt Alan Silvestri’s score for much else, usually throwing in pop only during a few key “Guardians of the Galaxy” crossover moments are happening, referencing the all-but-constant stream of ‘60s and ‘70s songs that figure into that universe as actual story points. So it’s inevitable that even more fanman and fanwoman scrutiny goes toward what few picks there are from Jordan (presumably) in a film like this one that doesn’t much sweat to the oldies. “Dear Mr. Fantasy” is a resonant enough pick that maybe he merits his paycheck after all.

The obvious tonal explanation: Traffic’s observation that (to quote the Eagles) most of us are sad is apropos, coming on the heels of an introductory scene that reminds us half of civilization went ashes-to-ashes at the end of the previous movie, a mass disappearance overseen by the malevolent rapturer Thanos (Josh Brolin). The literal explanation, clear to anyone who’s a little bit up on the Marvel universe: Glum Tony Stark (Robert Downey Jr.) and Nebula (Karen Gillan) are playing flick football and awaiting certain death in deep space aboard the Benatar, where the Microsoft Zune belonging to absent Peter Quill (Chris Pratt) would have to be their source of tuneage.

(As “Guardians” viewers will recall, Quill/Star-Lord had his mother’s precious Walkman full of vintage songs smashed to bits in “2,” but quickly inherited from the late Yondu an equally dated and emotionally fraught piece of music hardware, the playlist of which will figure into “Guardians of the Galaxy 3,” James Gunn has promised.)

That leaves only the symbolic question to consider: Who’s Mr. Fantasy, in the Marvel, not Dave Mason, universe? Well, he’s Iron Man, of course — who in this sequel merits a more somber theme than either Black Sabbath or AC/DC could provide. “Endgame” may be an ensemble piece, but by the end of it he’s achieved something close to Main Character status. Tony Stark is the class clown whose melancholia creeps up only sarcastically and around the edges — the troupe’s go-to entertainer as well as constant savior, who maybe can’t juggle being both the wisecracking kid and genius-father-knows-best figure forever. “Sing a song, play guitar, make it snappy,” sings Winwood, maybe referring — 50 years ahead of time — to Stark’s role as joker. “You are the one who can make us all laugh, but doing that you break out in tears.” Is Tony still an emotionally warped guy, despite all the domestication he’s been through since first appearing as a playboy billionaire in 2008? “Please don’t be sad — if it was a straight mind you had, we wouldn’t have known you all these years.” Maybe the filmmakers are trying to tell us that Tony is still so messed up, even Pepper Potts couldn’t reasonably say, “I will fix you.”

The needle is up almost as quickly as it was dropped: three hours doesn’t mean there’s time to fetishize a record (or MP3) collection. There are a couple of choices that don’t require a lot of overthinking— Steppenwolf’s “Hey Lawdy Mama” gives Stan Lee a chance to go out on a sleazebag cameo, and the Rolling Stones’ “Doom and Gloom,” from a few hits collections back, finally gets some sync love during a time-travel prep scene.

The most fun pick, though, is the Kinks’ “Supersonic Rocket Ship,” which doesn’t just push the obvious button in accompanying the scene where Rocket Raccoon and the Hulk travel to re-recruit Thor, but does actually describe the spirit of inclusiveness the “Guardians” franchise is meant to embody: “On my supersonic rocket ship, nobody has to be hip,” Ray Davies promised in the swingin’ jet age of 1967. “Nobody’s gonna travel second class / There’ll be equality / And no suppression of minorities / Well alright.”

The artists who gets the most audio time in “Endgame,” though?  Harry James (and his orchestra) and Kitty Kallen, who get a reprise of Jule Styne’s and Sammy Cahn’s end-of-WW-II-celebrating “It’s Been a Long, Long Time,” reprised from a “Captain America” movie for a twist on bringing the boys back home. It doesn’t seem like Steve Rogers’ taste, necessarily, but it’d be nice to think he took advantage of his time-travel sojourn to take in a Traffic show.

It’s the bittersweetly cheerful bookend to that bummer-iffic opening. “Dear Mr. Fantasy” resonates in part because it was one of the early songs to make the Summer of Love feel like winter was coming. That it can serve a similar, lightly ominous purpose in a comic book franchise laden with heartache in 2019 is an interesting addition to the zeitgeist in a year when we’re trying to decide whether resuscitating Woodstock is a good idea or not. Tony Stark, hippie jester in billionaire-industrialist-superhero’s clothing, we hardly knew ye.