Name: Senshi no Chikara wo Mezame Saseru Tame, Utau Rei [In Order to Awaken Her Senshi Powers, Rei Sings]
Original Air Date: March 13, 2004
Director: Takemitsu Sato
Writer: Yasuko Kobayashi

Important Plot Points:

Dark Mercury's first interaction with Nephrite

Sailor Mars Awakens