Sienna Hart hadn’t expected Christmas to look like this—standing in a Los Angeles recording studio in December, wrapped in a knitted scarf, sipping peppermint tea while staring at the words "duet with NSYNC" on her schedule.

The guys had been warm, funny, and loud in the best way. But JC?

JC had been something else entirely.

Soft-spoken. Focused. Unfairly handsome in a winter sweater. And the way he looked at her when she sang? Like he was hearing the Northern Lights.

They were recording a brand-new track for NSYNC’s holiday album—“Snowfall Serenade.” The producers wanted two distinct voices: JC’s smooth, velvety tone and Sienna’s bright, soulful edge.

The harmony was supposed to sound magical. No one expected the chemistry to be louder than the music.