Credit: Jeremy Deputat

Eminem and Rihanna: Why? He’s the reedy-voiced rapper from Detroit who defined MTV when the channel’s current demo was in diapers. She’s the smoky-toned singer from Barbados who defines the modern musical landscape of free-form hip-hop that sounds like dance-pop, or whatever the hell “Diamonds” is. He’s a confessional storyteller. She’s a tantalizing enigma.

Sure, they’ve partnered on a few songs—most famously the 2010 smash “Love the Way You Lie,” a track that helped define Eminem’s older-wiser-louder circa-2010s persona. But on the opening night of the duo’s joint-headlining Monster tour at Pasadena’s Rose Bowl Stadium, there was still that central question: Why these two together, right now? The ultimate answer, frustrating and thrilling: Why not?

The two stars opened with their 2012 duet “Numb” and shared the stage for collaborations both expected (“Love the Way You Lie”) and less so (B.o.B’s “Airplanes”). There’s a thrilling dissonance to the pair of them sharing space: Her, all wry grins and seductive body rolls in comfy-looking harem pants; him, all perpetual grimace and live-wire apoplexy in a punkish hoodie/baseball cap combo.

It’s too bad they’re not together for long. After a high-energy opening, Eminem ceded the stage to a fast-paced nothing-but-hits Rihanna -concert, with the singer racing through 17 songs in a little over an hour. She is a commanding presence onstage—there’s a catatonic charisma to the way she occasionally puts little or even negative effort into singing into the microphone. But somewhere between “What’s My Name?” and “Umbrella,” it started to feel as if the audience was singing half the song for her—it was like watching a ridiculously confident karaoke singer lead the world’s largest Rihanna sing-along.

As an entertainer, Eminem is both more energetic and more needy, and the show took on a looser vibe when he took over after the “Love the Way You Lie” handoff. Flanked by hype man Mr. Porter, he thanked the crowd for “making me feel at home” before launching into a set that included recent songs (meh), deep cuts (cool!), and finally, a medley of golden-era Slim Shady classics (incredible).

Rihanna gamely played the role of Dido on “Stan,” a thrillingly bleak Hall of Fame Em track that got megaphoned into a genuine banger for the arena crowd. Then Rihanna kicked off the show’s final act with a slow acoustic version of “We Found Love” before quickly seguing into an EDM-on–steroids version of “We Found Love.”

At one point, it all began to get a bit rambly; the night’s grand-finale encore, the tour-titling 2013 hit “The Monster,” came off as very energetic and very just-okay. Rihanna stood center stage, not so much dancing as gyrating gradually while Eminem raced around her, sweat pouring off his brow. It’s hard to say what brings these two very different stars together, but it’s fascinating to watch. Forget oil and water—this is gravity and anti-gravity. B+