Name: Kaette Kita Mamoru [Mamoru Came Back]
Original Air Date: May 22, 2004
Director: Masataka Takamaru
Writer: Yasuko Kobayashi

Important Plot Points:

Mamoru returns

Mamoru tells Usagi that in their past life, the world was destroyed because of their relationship

Mamoru promises that the planet will not be destroyed despite their relationship