The duo kicked off their drive with Billie's chart-topping smash, "Bad Guy." She harmonized while James sang along to the track. "This song is literally like two notes! Don't have to try," she assured him, shouting excitedly mid-song, "Ooooo, God, that shit is HARD!"
Turning down the music, Corden asked his guest which fellow musician she's met in the last year that she was surprised to learn was a fan of hers. "Billie Joe Armstrong," she said, still processing the shock of the news. "Justin Bieber," she then added, "who you were there for when we met."
"I was there!" Corden confirmed. "We were at Coahcella, we were watching Ariana Grande. Tell me about what he means to you." A shy Eilish put her hands to her face and let out an "oh, God!"
"Well, if I can be honest with you, for like all Coachella weekend, I was like, 'Don't surprise me with Justin Bieber. I can't take it. I can't have to do this show and Justin Bieber's gonna be there," she explained. "And so we go to see Ariana, and I was just standing there, and we were just watching the set, and I was rocking out 'cause I love Ariana, and I saw... 'Cause I know what Bieber looks like. I know his body language, I know how he stands, I know where he wears his pants."
"So I look over, and I just was like [eyes wide]," she went on. "And he just stood there! He didn't even come near me. He just stood like five feet away from me, perfectly still, and he just was like [staring]. And he had the face mask, so all I could see were his eyes, but they were his eyes."
"They were his ocean eyes," Corden quipped, but Eilish jokingly reprimanded him. After all, Bieber is a married man.
The two then broke out into JB's first hit, "Baby," without a track and without missing a beat. They transitioned seamlessly into Billie's "All the Good Girls Go to Hell."
Lowering the music again, Corden asked his guest, who just turned 18, when she first started writing songs. "If I say 6, it sounds stupid because there's like no way. I wasn't like sitting down writing a whole song," she explained. "I was making up songs." She was also a whiz at the ukulele and told James he could be, too. Conveniently, he had one in the backseat and handed it over to Billie. The first song she learned to play was "I Will" by The Beatles. She sang him a snippet of it, and he was floored.
"I wrote my first song on the ukulele with my friend. I think I was 7," she said, before going into that very song -- and a few other originals. The two then broke out into Eilish's "Ocean Eyes," a song that "gave me my life."
"That song is the reason that I have the life I have," she said of the track she wrote at 13.
Billie recalled growing up with parents who could not get her to go to bed if she and her brother, Finneas, were up making music and messing around, "because making music is messing around." Her debut album and EP were both recorded in his tiny bedroom, "no booth, no studio." She still lives there today, so Corden wondered if they should just swing by and check out the house. She was game.
Eilish gave the comedian a tour of their subdued and adorable home. Making their way down the hall -- which included one bathroom and Billie's "very red" room -- they got to Finneas' room, "the belly of the beast." She showed him a white board with the names of all the songs they've written together, explaining that the filled-in circles next to each track name were once empty.
"Do you feel like being at home and still living at home has grounded you in the last 12 months?" he asked, to which she replied, "Probably more than I realize. Also, just like being around my parents all the time. And even on tour, my parents are there. My entire family, besides my dog and my cat and my spider... I have a spider. You wanna see it?"
James said yes, but he certainly didn't mean it. After she left the room to go fetch her pet, he nervously confessed he wasn't "good with spiders." At the first glance of the little guy's eight, hairy legs, Corden sprung up from the bed. "Why would you do that? Why would you have such a thing?" he asked. Eventually, she got the comedian to hold her pet. He reacted as you'd expect -- glaring at the camera people and begging for help with bulging eyes. The musician left the room in hysterics, leaving James to hang onto the spider for just a few more minutes. "I genuinely felt like we were becoming friends," he stated angrily. "I don't like it at all. I don't like it in the slightest."
After James composed himself, Billie recalled the moment her brother promised her, "I'm gonna make you the biggest pop star in the world." She said they both laughed at the sentiment, which Finneas made in the beginning of 2015.
We also got to meet their mom! She walked in and confirmed Corden's question if the two kids would sit and scream for her to bring them food when they were recording music. She said Billie would ask for a burrito, and Finneas would request a grilled cheese sandwich. "And I'd lay in the bed, and I'd listen to whatever they did," she said proudly.
"People are like, 'How does it feel? She's at Radio City!' and I was like, it blew my mind when she sang at the Hyatt, you know what I mean? It blew my mind when I sat here and heard Billie and Finneas play a song. Like, it's never not been mind-blowing," she said. "It's almost like it's been four years of 'oh, my God.' It's not like I can never feel it in the moment because the whole experience is so crazy. For a while, they'd write these songs, and I'd be like, 'When is it gonna come out 'cause everyone in the world has to hear it!' And now, everybody hears it, and I can't describe it 'cause it's too amazing."
All three people were in tears.
The epic episode concluded with Billie and James back in the car, belting out "When the Party's Over."