What happens if you bring Meloetta to the Cafe in Castelia?

Question

In Black/White 2

Answer 1

Meloetta Event
If you bring the event Pokémon Meloetta to the Café within Slim Street, the musician playing in the café will tell you about Meloetta and how it once lost its voice. Here, it will teach you the move which allows Meloetta to change form. - Serebii

(This move is Relic Song, BTW)
So, that is what shall happen.

Hope I helped. :)
Mr. Source.