If there is one song you will be left struggling to get out of your head after watching “You’re Cordially Invited,” it will be “Islands in the Stream.” There are no less than three versions of the Dolly Parton and Kenny Rogers song sung by the cast in the film — but are they actually singing themselves?

Now streaming on Prime, the movie — directed by “Forgetting Sarah Marshall” and “Neighbors” vet Nicholas Stoller — centers on two families who unknowingly have weddings double booked at one tiny inn. One is for Jenni (Geraldine Viswanathan), booked by her dad Jim (Will Ferrell). The other is for Neve (Meredith Hagner), booked by her sister Margot (Reese Witherspoon).

Eventually, Margot and Jim agree to try and share the space for the weekend. They trade off between taking the outside and indoor event space, but when a massive rainstorm hits, Jim and his family are forced to crash the indoor festivities.

Determined to make the best of it, Jim pushes through and gives his toast, as well as a performance of “Islands in the Stream” — with his daughter. Yes, it is weird. Yes, everyone but him sees it is weird.

But also yes, that is actually Will Ferrell and Geraldine Viswanathan singing together. According to the credits, the two actors really did sing the song together — perhaps unsurprising, considering Viswanathan starred in “Emo: The Musical” and Ferrell has had plenty of musical performances in past projects.

For the record, Jim only chose that song for him and his daughter because it is the song he and his wife would sing to get Jenni to sleep as a baby. Viewers are even treated to a slower, more tender version of just Ferrell singing it during a montage (again, still really him).

Later, as the credits roll, we are treated to one more performance of the song, this time by Ferrell and Reese Witherspoon. And yes, that is really her singing too, as she is billed for the cover in the song credits.

Once again, it is not crazy surprising, given that Witherspoon’s pipes are on full display in the “Sing” films, not to mention her Oscar-winning performance in “Walk the Line.”

In short: the actors in “You’re Cordially Invited” really are belting their hearts out. Uh-huh.

“You’re Cordially Invited” is now streaming on Prime Video.