Serena and Darien were trapped in the Star Light Tower. When they saw that there was no escape, they transformed into Sailor Moon™ and Tuxedo Mask™. Just as each learned the other’s true identity, Tuxedo Mask was injured trying to save Sailor Moon. The shock made Serena realize that she is the Princess of the Moon.
Darien was Serena’s true love, a long, long time ago.