As the
story dictates, 1000 years ago was the Silver Millenium on the Moon. It
was a joyous time for everyone. The ruler of the Moon Kingdom was Queen
Serenity and she had one daughter, Princess Serena. Princess Serena had
fallen in love with a human Prince from Earth, Prince Darien and spent
most of her time staring at the earth.
The Moon People watched over the Earthlings until a conflict arose between
the two peoples. The Moon people thought they wanted to be able to
live for 1000 years, as they did. A powerful creature named Queen
Beryl came out of nowhere and manipulated the people of the earth.
She wanted to conquer the universe and the first target was the moon.
Prince Darien was asked by Queen Serenity to stay and defend the moon.
They had one last dance that night and told each other what they meant
because Darien might not come back.
One night, before the battle against Earth would begin, Serena gave her Star Locket to Darien to make sure he would never forget her and what they had together.
Finally the time came when the Earthlings attacked
(with Queen Beryl leading them). The Prince fought on the side of the Moon
to protect his love. This fight came to a bitter end for the Prince and
the Princess were both killed. Queen Serenity, being the loving mother
she was, wouldn't let the enemy end her daughter's future that way.
Using the power of the silver imperium crystal, she mananged to trap the
enemy and the people of the moon kingdom. If she had destroyed them,
she would have also destroyed Serena and the others. She still had
enough strength left to send the enemy and the people of the moon to be
reborn to a new future on Earth. Queen Serenity knew she must sacrifice
her kingdom if any of her people were to live on. She told Luna and
Artemis, the guide cats, that if the Negaverse ever broke free and tried
to repeat what they did on the moon, they would know what to do.
Her last dying words were, "On behalf of the moon, you will be free again."
The Present Part I
The romance between Serena and Darien has been a very jumpy one since
the start of the show. In fact, they hated each other.
The 
two didn't
really hit it off that well till much later in the first season of Sailor
Moon, when the past is brought to light. Throughout the first season we
get hints as to who is who and how the past ties into the story but never
enough to make a clear decision. The romance really begins to show at the
beginning of Zoisite's quest to get the Rainbow Crystals and the Silver
Crystal. This is when Darien first discovers he is Tuxedo Mask and starts
to understand the dreams he is having. In one of the episodes an artist
(Peggy Jones in English) asks to have Darien and Serena be her models.
In her studio Serena discovers a picture called "Till We Meet Again." She
shows it to Darien, who realizes that it is a scene from his own dreams:
the Moon Princess giving her moon locket to Prince Darien, so they would
always remember each other.
Darien/Mamoru continues to get the visions till the Moon Princess is
discovered in the 
Episode 30 "Crystal Clear Destiny." In this episode the
Moon Princess is revealed and the two lost lovers from long ago are freed.
Darien is captured by Beryl and pitted against the Scouts, but his love
breaks through in the end and he manages to not destroy Sailor Moon, but
save her. He is finally totally brainwashed by Beryl for Episode 40 "Day
of Destiny."
Darien attacks Sailor Moon in the form of Prince Darien, but is freed of the evil spell by the love deep inside opened by the Moon Locket song, which only played for the two lovers destined to be together.
The Present Part II
In Sailor Moon R from the first episode that Reeni
arrives, Serena and Darien's relationship falls on some hard times. It
all begins with a nightmare that Darien gets one day about Serena in the
future. It warns him to keep away from her or she will die. She happened
to bump into him after getting another 30's grade on a math test, and he
dumped her on the spot. She is heartbroken and tells her friends
that their relationship was over. Later on, Serena gets the
same sort of dream. Being the feeling and sharing girl she is, she tells
Darien about the dream she had. They talk and realize it was all a trick
and get back together. In Sailor Moon R the Movie the romance is once again
threatened, this time by one of Darien's old friends, one he had met as
a young child when he was in hospital, Fiole. Fiole will not let anyone
stand in his way to repay the humans for having left Darien alone for so
many years. 
He (Darien) does not realize the important element that Serena
plays in his life now. Eventually Mamoru comes to her rescue by attacking
Fiole. Fiole does not understand why he would want to protect such a deceiving
girl and Darien explains to his friend what Serena means to him. She uses
the power of the Silver Crystal to break the spell cast over Fiole and
to save her friends. The use of the crystal would normally kill her (as
it did her mother) but Fiole, a true friend, helps her by giving Darien
the nectar of the flower of life and his kiss revives her. 
The Present Part III
In Sailor Moon S Darien and Serena's relationship does not take precedence
over anything else in the story. The two are simply going out the way it
was meant to be. They see and love each other, as a 
couple
should. In Sailor Moon S the Movie, Serena and Darien's relationship is
put back into the light when Luna asks Serena about love, how love works
and what the feelings mean. She explains with Darien as her example. Throughout
the rest of the series, the relationship between Serena and Darien comes
up briefly but never more then is needed. It is known that they date and
love each other very much and that is how it remains. 
The End??
As the Manga dictates, the Sailor Moon Romance Saga ends with the marriage
of Serena and Darien. These two star-crossed lovers from the past
finish what could not be done on the Moon Kingdom 1000 years ago. The wedding
turns out to be the ending and finish all of the Serena/Darien relationship
in Sailor Moon, at least in the Manga. 
In the Anime, the series and romance ends a little differently, with
no creation of Crystal Tokyo and no wedding, but just two lovers, kissing
under the full moon. And that is the story of the romance between
Princess Serena and Prince Darien. 
Whew! That was a long and confusing story, huh? When you
watch it, you have to know what time period it is because in the beginning,
Serena hates Darien because of his minus-0 personality but likes Tuxedo
Mask. Unfortunately, Raye does also. Much later, Serena starts
to like Darien but he is turned and brainwashed onto Queen Beryl's side
who wanted him 1000 years ago and hated Princess Serena for taking him
away from her even though she was much older. This whole time, Darien
has no memory and that's bad for Serena because he never remembers anything.
I like to separate Sailor Moon into three parts: 1.) Queen Beryl's
first four bad guys which ends after Zoisite gets the seventh rainbow crystal
while the sailor scouts are forming. 2.) The past is revealed, Prince
Darien is brainwashed, He and Malechite battle to get the imperium silver
crystal, and Sailor Moon and Darien get together. 3.) Ann and
Alan arrive and create a conflict among themselves, Serena, and Darien.
They are a couple and then break up when Reeni arrives. The time
period when Darien dumps Serena, the Dark Moon sisters and Rubeus, the
other dark moon kingdom, and Serena and Darien finally get their happy
ending back.