Fifth Harmony Crashes Proud Harmonizer Sam Smith's 'Carpool Karaoke'

Earlier in the ride, the "Stay With Me" singer declared that he's a "Harmonizer," one of the girl group's loyal fans, and "the woman in me is on fire" when he listens to their song "Work from Home."

"I think it's my wedding song. I want to walk down the aisle to it in heels," he said, before sharing some more wild details about his future nuptials. "For my wedding, I have a whole plan for it. I want a funeral for my single self before my wedding. I'm in a coffin and there's all these Power Point presentations about my single life and how fun it was. Everyone's crying, and then I rise from the coffin. Everyone takes off their black clothes and they're wearing white, and then the wedding begins."

After hearing this, Corden made a call to a secret friend, saying their car performance of the 5H song "could be elevated somehow." The look on Smith's face when he realized what was happening was just priceless.

After losing his cool when they piled into the car, the group all sang "Work from Home" together. "That made my year," Smith declared afterwards.

During the 15-minute video Smith and Corden also sang "Too Good at Goodbye," "Lay Me Down," "I'm Not the Only One," "Money on My Mind," "Stay with Me," and "Pray."

Corden also joked about Smith's impressive weight loss, saying, "We started the same diet at the same time and now I'm just on my own, doing it for the big guys. There's only two ways to rectify itself, either you get big or I lose weight."

"I'm 100% down to get fat again," Sam replied. "My plan is to keep my weight down for 10 years and then I'm just gonna live in caftans. No pants, glass of wine. And I just walk around with my Oscar and go through photo albums."

He also revealed he was star struck when he met Ryan Gosling -- "Oh my god, I was dead," Smith explained, "He's beautiful!" -- and how he got caught skipping school for a Lady Gaga concert.

Watch more recent "Carpool Karaoke" videos below.