Spike likes Rarity, Sweetie Belle likes Spike, Rarity doesn't know about ether. Until Rarity starts returning Spike's feeling causing a war between Siblings.
While Twilight is away at a sleepover at Rarity's boutique, Sweetie Belle spends the night with Spike at the library. Spike plans to use this to his advantage, and finally tell Sweetie Belle that he loves her.