After Usagi was revealed as the Moon Princess, an injured/dead Mamoru was swept away by the Dark Kingdom. Queen Beryl rid him of his memories and brainwashed him into the Dark Prince Endymion. He lived to serve Beryl, and to retrieve the ginzuishou at all costs.
After Mamoru had been taken away by the Dark Kingdom, Usagi worried about what could have happened to him. She spent time admiring and listening to the musical pendant from Tuxedo Kamen. One day, she and the others had to battle a youma in a hair salon. The youma had been trying to find Sailormoon's real identity through a strand of hair she had shed during a previous battle. The youma mistook Minako for Sailormoon. The real Sailormoon arrived to fight.
She attempted to use "Moon Healing Escalation" to restore the youma to its previous human form, but a rose hit the Moon Stick and knocked it out of her hand. Sailormoon saw Tuxedo Kamen at the far end of the room, sitting in a chair. "Tuxedo Kamen!" she called out. He replied that he wasn't Tuxedo Kamen, he was infact Endymion, from the Dark Kingdom. He demanded the ginzuishou from Sailormoon. Confused, Sailormoon panicked and ran from the youma and Dark Endymion.
At that point, Minako awakened and became Sailorvenus. She and the other senshi found Sailormoon as the youma had almost caught up to her. She was finally able to perform Moon Healing Escalation and restored the youma back into the hair dresser. The senshi couldn't believe that Mamoru had been turned to the dark side.
For a few episodes, one would have to be skeptical as to whether the brainwashing of Mamoru had fully worked. Many times, Kunzite wanted to kill the senshi, but Dark Endymion stopped him. In episode 38, a youma sent an ice spear towards Sailormoon and Sailormars, but Dark Endymion, instead of throwing a black rose, threw a red rose and stopped the spear. He said that he didn't want to defeat the sailor senshi just then. Then, the rose decayed to black, and a few petals fell to the ground.
In episode 39, Sailormoon was fighting on an ice skating rink; not exactly her battle setting of choice. The youma was about to get her when suddenly Dark Endymion swooped in and saved her. He said something about not liking how the youma picked on innocent people. Now we have to realize that Mamoru's brainwashing hasn't completely worked - he's evil, and still manages to save Usagi on a regular basis. In episode 40, Dark Endymion even told Usagi's family, who was in danger, to escape while he held off a monster he had unleashed.
In episode 41, Sailormercury and Sailorjupiter met Dark Endymion in an alley, and Jupiter challenged him while Mercury escaped with Urawa Ryo. He proved to be too powerful for even the very strong Sailorjupiter, and had her up against the wall of the alley. He drew his arm back, ready to punch her. Sailorjupiter screamed, but Dark Endymion stopped just inches away from her face. He laughed and said that she was no challenge, then proceeded to leave the scene. In my opinion, if he was really truly brainwashed, he wouldn't have cared about all these "challenges" and not liking the way the youmas acted. If the brainwashing was complete, he wouldn't have saved the senshi, and he would have punched Sailorjupiter.
Later in the same episode, Sailormercury challenged Dark Endymion. She managed to steal away the kurozuishou that he had been using to capture victims. She smashed it and freed those inside. Sailormoon arrived and attempted to use Moon Healing Escalation on him. He began to remember some things - Mamoru making fun of Usagi and such. Beryl's brainwashing had not been complete. They successfully changed him back into Mamoru. But Sailormoon's efforts were futile - the Dark Kingdom's black portal appeared and took him away once more. But Usagi would never give up.
In episode 45, the senshi travelled to Point D in the Arctic to find the Dark Kingdom. Beryl wanted to face Sailormoon alone, so she sent out her most powerful youma to kill the other senshi. One by one, they sacrificed themselves to save Sailormoon. She was alone to face Queen Beryl.
When she arrived at her destination, she found the odds against her. Out of the shadows came Dark Endymion. This time the brainwashing was complete; he was now ruthless, cold, and obedient to Queen Beryl. His eyes were completely hollow and void of feeling. Sailormoon looked at him in shock. She once again attempted Moon Healing Escalation, but it did nothing. It simply passed over Dark Endymion like air. He stood up and invoked a black rose. "Sailormoon....die." He tossed the rose at her. It extended into vines with many roses and began shocking her. She screamed in pain, remembering back when Mamoru used to call her "Odango Atama". The roses finished their job and she collapsed to the ground.
The cruel Dark Endymion approached Sailormoon and kicked her with so much power she went flying backwards. He then picked her up and began choking and shocking her. He reveled in her screams of agony. He then threw her to the ground. She landed only a few inches from her Moon Stick, but she had no real will left to grab it. She just stared at it. Dark Endymion drew his sword and prepared to make one final blow. Sailormoon then remembered the death of her friends - how they had died so that she could live on and fight.
She used her tiara and caught him in his midsection. The blow shocked him enough to stumble and fall to the ground. After a few moments, he rose once more, ready to kill Sailormoon. This time, she kneeled and held out the star pendant, pleading with him to remember how it was before - how it was when he was just baka Mamoru-san. She pleaded with him to touch the pendant. He did, and memories flooded through his mind. He remembered everything. "Usagi..."
Furious, Queen Beryl sent a rock spear at the couple, in attempts to kill them both. Mamoru threw a rose at the spear, which broke the spear into many pieces but did manage to strike Queen Beryl in the chest. Her body began to crumble, and she said in disbelief, "Is Endymion's love for this girl destroying my body?" Mamoru took a few spears in the back, protecting Sailormoon, and collapsed to the ground. A rose fell out from within his armor - a red rose. He told Sailormoon to leave before the cavern collapsed on them. She held him; the rose then fell apart and Sailormoon knew that he had died saving her. She began to cry over his body...
She knew that she had to leave and do battle. She leaned in close to Mamoru's body, ready to kiss him. She remembered her friends had all died without kissing the boys they loved, so she stopped. "I can't..." She got up and left the cavern, ready to use the ginzuishou to defeat Queen Beryl and Queen Metallia.