Camila Cabello has a "dirty secret" about her summer, and Shawn Mendes knows all about it.

The Fifth Harmony member and the pop singer teamed up for the dramatic acoustic track "I Know What You Did Last Summer," which is all about a girl cheating on her boyfriend.

"He knows dirty secrets that I keep/ Does he know it's killing me?/ He knows, he knows," Camila sings before they both join in on the chorus. "I know what you did last summer/ Just lied to me, "There's no other"/ I know what you did last Summer/ Tell me where you've been/ I know what you did last Summer/ Look me in the eyes, my lover."

Their voices blend in perfect harmony delivering a pure pop track that will no doubt appeal to both of their massive fan bases. The song, which is the lead single off Shawn's Handwritten Revisited reissue (Nov. 20) was written backstage at Taylor Swift's 1989 World Tour, where Shawn was an opening act.

The two instantly clicked and wrote the chorus for "IKWYDLS" backstage at the show before going into the studio, where they actually improvised some of the words.

"The bridge was completely improvised where it's a conversational ad-lib moment," Camila told New York's Z100 radio station. "We were in the studio and we were separated by two pieces of glass and we could hear each other in the headphones, and there was a piano melody in the background. Keeping in mind the storyline and narrative, we just sang to each other."

Whatever they did worked, and the two couldn't be more excited, and nervous, for the song's release.

"I think when you're so invested in something, it's our story, so when it's brought out for people to hear, it's our child being born," Camila said. "And it's like, are people going to like our child?"