What I remember about it is that Kurt is boarding at Dalton and has to use the communal showers, which he doesn't want because he is afraid of anyone seeing him naked.
Blaine helps him overcome his issues by showering with him (or something).
I hope someone can help me find this.
P.S. Mods, I hope the tags are right.
*found: "the mirror's distortion" by pyroclastic (link in comments)