Cassandra Truth: Alec doesn't believe Valentine (who's actually Magnus inside his body) when he tries to tell him about the body-swapping spell, initially.
"Freaky Friday" Flip: Magnus and Valentine becomes victims of a body-swapping spell put on them by Azazel.
Internal Reveal: Isabelle and Simon learn that Clary and Jace aren't siblings.
Raphael's clan also finds out that Simon is a Daylighter.
Nice Job Breaking It, Hero: Alec kills Azazel, leaving Magnus and Valentine trapped in one another's bodies.
Screw the Rules, I'm Doing What's Right!: Inquisitor Herondale, ignoring the Clave's orders, wants to have Valentine executed when it seems that he won't reveal the Mortal Cup's whereabouts, unaware about the body-swapping spell either. Fortunately, Alec stops her in time just as he learns that Magnus (who is Valentine inside his body) has captured Jace.
Wham Line: Valentine telling Jace that he's a Herondale, a a respected and royal-like family line, making him Inquisitor Herondale's grandson.
