Unfortunately, a conflict had arisen between Earth and the Moon. The Moon Kingdom believed that the earthlings wanted to be able to live for one thousand years, just as the Moon Kingdom did. So the Earthlings were not to be trusted. This did not help the young lover's relationship. They often would have to meet secretly and their connection grew more and more every time they met.
One night, before the battle against the Earth would begin, Serenity gave her Star Locket to Endymion to make sure he would never forget her, and what they had shared together. Finally, the time came when the Earthlings attacked. Queen Beryl led the attack against the Moon. The Prince fought on the side of the Moon to protect his love. The fight came to a bitter end for the Prince and Princess were both killed. Fortunately, Queen Serenity (Princess Serenity's mother) was able to destroy the enemy and send the people of the Moon to a new life on the future Earth.
Now in the future, the romance between Serena (Usagi) and Darien (Mamoru) had a very bumpy start. The two didn't really hit it off well till much later, when the past is brought out into the light. The romance begins during Zoisite's quest to get the Rainbow Crystals and the Silver Imperium Crystal. This is when Darien first discovers he is Tuxedo Mask and starts to understand the dreams he is having. At one time, an artist named Peggy Jones asks to have Darien and Serena be her models. In her studio, Serena discovers a picture and shows it to Darien. He realizes that it is a scene from his own dreams, the Moon Princess giving the Locket to him from so long ago.
Darien continues to have the visions till the Moon Princess is discovered (which we find out is Serena). Once the Moon Princess is revealed, the two lost lovers from so long ago are freed.
Darien is soon captured by Queen Beryl and pitted against the Sailor Scouts. Only his love for Serena breaks through in the end and he manages not to destroy Sailor Moon, but instead to save her. Beryl captures Darien again and totally brainwashes him. Darien attacks Sailor Moon in the form of Prince Endymion. He is freed of the evil spell by his deep love inside upon hearing the Moon Locket song, which only plays for the two lovers destined to be together.
Serena and Darien's relationship falls on some hard times when Rini (Chibi-Usa, their daughter from the futre) arrives on the scene. It begins with a nightmare Darien has one night about Serena in the future. It warns him to keep away from Serena or she will die. The very next day he breaks up with her, leaving her heart broken. Later on, Serena has the same type of nightmare. Being how she is, she tells Darien about the dream. They talk and start to realize the dreams were all a trick and get back together. The Black Moon shows itself and are defeated by the love of the two.
The romance is again threatened by an old friend of Darien's, Fiore, from his childhood when he was in the hospital. Fiore will not let anyone stand in his way to repay the humans for having left Darien alone for many years. What Fiore does not realize is the important element that Serena plays in Darien's life now. Fiore tries to kill Serena and Darien steps in front of her and takes the full blow. Fiore takes Darien to heal and the Scouts go out to space to save him. Once there, the Scouts are attacked and eventually Darien is able to come to Serena's rescue by attacking Fiore. Fiore doesn't understand why Darien would want to protect such a decieving girl. Darien explains to his old friend what Serena menas to him. Serena uses the power of the Silver Crystal to break the spell cast over Fiore, and to save the Earth. The use of the Crystal would normally kill her (as it did her mother, Queen Serenity) but Fiore, being a true friend, saves her by giving Darien the nectar of the flower of life. Darien's kiss revives Serena.
The two eventually get married and the two star-crossed lovers are able to finish what could not be done in the Moon Kingdom one thousand years before. They rule Crystal Tokyo together and have one daughter, Rini.