On Valentine's day, Usagi made a muffler she wanted to give Mamoru but was sadden to find out he was engaged to be married to a childhood friend named Hina. Tuxedo Kamen's true identity is eventually discovered by Usagi. When Tuxedo Kamen is wounded by Kunzite, Usagi tried healing him and cried tears and became the Moon Princess. Mamoru was preparing to move to London with his fiancee, Hina. Hina was concerned about his strange behavior, and discovered a strange pink handkerchief in Mamoru's drawer. She realized how close he and Usagi had become, she sent him to London to study on his own. After saving Usagi's life once again, he inadvertently caused one of the Shitennou to begin to remember the past. While in London, he was contacted by Zoisite, who was convinced Mamoru was the reincarnation of his Master, Prince Endymion. Indeed long ago, Usagi was Princess Serenity of the moon and Mamoru was the Prince Endymion of Earth. Their love ended in tragedy and was the cause of the end of the Silver Millennium, a time of peace. Using his music, Zoisite coerced the Shitennou and Mamoru to remember their pasts and how Endymion's love for Serenity had caused the devastation of the planet. When Mamoru returns from London, he tells her he returned to prove that would never occur again. He then asks her for the muffler she made for him.
Once his memories of his past and present lives were restored, he assumed his Endymion form to help the Senshi. When Beryl tried to convince Mamoru, to come to the Dark Kingdom, the trauma caused Princess Serenity resurfaced from within Usagi and went on a rampage. While she may have driven Beryl back, Mamoru was taken to the Dark Kingdom. Not only did he want to save the remaining Shitennou, he wanted to understand more about the link between the Ginzuishou and Metalia. He learned whenever Serenity used the Ginzuishou it made Metalia stronger. He tried his best to protect Usagi from the dangers of overusing the Silver Crystal. Usagi could barely keep Serenity's power suppressed, knowing Endymion was trapped and in danger in the Dark Kingdom. Beryl had a draining stone implanted in him, and whenever he thought of Usagi part of his life was drained away into a painting. But, no matter what he did, he couldn't stop loving Usagi. He also felt guilty of his behavior in the past life, and wanted to make amends by preventing the tragedy from repeating itself. Endymion took Metalia into his body and was succumbed to the dark power. As Dark Prince Endymion, he smashed Sailor Moon's Moonlight Stick. He almost killed Sailor Jupiter and Mercury when Sailor Moon was forced to kill him to stop Metalia. She did so with the help of Mamoru's spirit and Endymion's sword.
In the final battle, Princess Sailor Moon released a wave of total destruction from the power of the Ginzuishou. Endymion appears before her in a desert and she rushes to him, upset because she was unable to keep her promise not to destroy the world. He tells her she can use the Ginzuishou's power to save the world. He wonders if this is what they need to do in order to bring an end to the past life. He apologizes for not being able to save her. Endymion slides a ring onto Serenity's finger and kisses her. The Ginzuishou restores the world, leaving the Prince and Princess lying lifeless on the sand. Princess Sailor Moon appeared to them and told Usagi that as long as her friends remember her, she can live. Mamoru tells Usagi to go on and forget about him, but she refuses. The Shitennou arrive, carrying the painting into which Beryl had drained his life. They tell him to go as well and live in the present, but if something happens they will surely come to help him. Mamoru takes Usagi's hand and the two of them run forward, into a bright light. In the Special Act, four years had passed and Usagi and Mamoru were to be wed but Usagi called it off, after overhearing Mamory said if their relationship would end, it should just end. Mamoru said this because he felt Usagi was concerntrating too much on the wedding ceremony than their actual marriage. Queen Mio kidnapped him and hypnotized him for him to marry them. The Shitennou did indeed come to Mamoru's aid as promised, but when fighting Sword and Sheild, he asked them not to interfere and he defeated both Youma. In the end, he married Usagi. In a vignette part of the Act Zero DVD, Usagi asked Mamory for the origin of Tuxedo Kamen. He told her that when he was young, he got bruised and was helped by an older Tuxedo Kamen who gave him a ban-aid. Supposedly, Mamory had the bandage from that time, but then told Usagi it wasn't true. But his Tuxedo Kamen idol was watching them.