George Clooney and Julia Roberts Crash Gwen Stefani's 'Carpool Karaoke'

The Oscar-winners brought some formidable star power to the Voice coach's 'Carpool Karaoke' on Wednesday.

Gwen Stefani and James Corden teamed up for another installment of "Carpool Karpool" on Wednesday's Late Late Show, and they picked up some unexpected A-list passengers: George Clooney and Julia Roberts.

The No Doubt frontwoman and the talk show host were having a perfectly lovely drive through Los Angeles -- singing Stefani's classic hits and talking about emoji -- when they ran into a little snag. It seems, for one section of the "carpool" lane Corden supposedly drives on, four passengers are required.

WATCH: James Corden Reveals His Dream Carpool Karaoke Guests

Corden wasted no time, hopping on the phone to call one of his mates. As it turns out, Clooney had nothing better to do that day than help Corden get to work on time, though he joked that the late-night host is a “pain in the ass.”

However, Clooney by himself still wasn't enough, so the Oscar-winner had to jump on the phone himself and convince one of his own friends to join the carpool, calling up his Money Monster co-star, Roberts.

As the actress joined the crew in the SUV, it was readily apparent that "Carpool Karoke" has never had this much star power. It was equally apparent that someone with Clooney's reputation of class and sophistication looks hilarious while singing along to "Hollaback Girl."

WATCH: Adele Perfectly Nails Nicki Minaj Rap on the Most Incredible 'Carpool Karaoke' Ever

The 54-year-old star claimed to know what the title of the song means, explaining, "'Ain't no hollaback [girl]’ -- meaning if some guys are on a construction site and they're yelling at a girl, she's not gonna turn back. She's not that kind of a girl. Am I close?"

"I think that that's pretty close," Stefani said, coyly. "It's your interpretation of the song. I don't want to, like, project what I thought onto you."

"Well, it would help to know what you thought," Clooney responded dryly.

"The real meaning behind that song, the real inspiration behind that song, I would never say," Stefani revealed, much to the chagrin of everyone in the car who immediately pressed her to spill the beans. "All I'll say is: I won."

"This s**t's bananas," Clooney replied. "B-A-N-A-N-A-S."

WATCH: James Corden and Jennifer Lopez Hilariously Text Leonardo DiCaprio in New 'Carpool Karaoke'

After riding for a bit, Corden, Stefani, Clooney and Roberts wrapped up the surprising segment with a massive group performance of Queen's 1977 anthem "We Are the Champions," because apparently, Corden wanted to set the bar as high as possible for future installments.

Prior to Clooney and Roberts’ surprise appearance, the most amazing "Carpool Karaoke" moment in Late Late Show history had to be when Corden texted Leonardo DiCaprio from Jennifer Lopez' phone, pretending that the "Feel the Light" singer was looking to party. Check out the video below to see the epic prank – and the Oscar-winner’s hilarious response.