Métamorphose means "metamorphosis" or "transformation". It's a beautiful and dramatic word that evokes a profound change, much like a caterpillar turning into a butterfly.
In this song, the singer describes the emotional turmoil of loving two people. She sings, "Mon coeur se métamorphose" (My heart transforms), using this powerful image to express how her feelings dramatically shift from one person to the other, as if her heart is constantly changing its very nature.
Have you ever tried to juggle your feelings between two people and completely lost the plot? That is the exact chaos in Stéphane's track. Right from the jump, she confesses: "J'ai l'impression d'être deux / J'en aime une puis j'aime l'autre" ("I feel like I'm two people / I love one, then I love the other"). She even shouts out non-monogamy directly with "Polyamour, c'est le feu / Mais c'est toujours une de trop" ("Polyamory is fire / But it's always one too many"). It is messy, honest, and wildly dizzying.
The real hook here is location, because her feelings literally flip depending on where she stands. In one line she is smiling on the streets of Montreux in Switzerland, and by the next she is traveling across Paris from Montmartre (the famous hilltop neighborhood) to Jussieu (down in the 5th arrondissement) wanting to offer a rose. Then comes the blunt, funny kicker: "Je t'aime ici mais là-bas / Je ne t'aime pas / Et vice versa" ("I love you here, but over there I don't love you / And vice versa"). Behind all the back-and-forth jokes, though, she admits she is just faking control ("Je fais semblant de gérer"), shaking with stress, and oddly tired of being loved at all.