The Harem Plan is a plan that was initially proposed by Lala and her costume robot, Peke, to Rito after Rito's confession of love to Lala (in Chapter 162 of the original To LOVE-Ru series). However, since Rito is also in love with Haruna, Lala decides to share Rito with her best friend. In her plan, she wants Rito to confess his feelings to Haruna (since Lala secretly knows that Haruna reciprocates his feelings too) so that they can all live happily together, since if Rito marries Lala and becomes king of Deviluke, Earth laws will no longer apply to him, making polygamy legal.
In To LOVE-Ru Darkness, it was revealed that Momo is also in love with Rito, but does not want to intervene in Rito and her sister's relationship. However, when Lala told Momo about her Harem Plan, this gave Momo hope to have a compromise where Rito can love Momo as well. She creates another version of Lala's Harem Plan so that all of the girls who are in love with Rito may be happy, including her, without hurting anyone. Momo's idea is to not only make Rito marry Lala and then have Haruna as his concubine, but she also wanted every girl who has feelings for him to become his concubines as well, with Lala as his legal wife. The girls included in Momo's Harem Plan are herself, Lala, Haruna, Mikan, Nana, Yui, Yami, Run and Rin, while Mea, Risa, Kyouko and Dr. Tearju are also under consideration to be part of the plan.
While Momo reasons, and does believe, that the Harem Plan is the perfect solution to guarantee the happiness of all of Rito's love interests, there is a hint of greed in her plan to use other girls to make a harem for Rito so that she may be included as well, as she doesn't care if she is second or third as long as she is loved by Rito. She is including not only already established love interests into her plan, but she is also on a lookout for other potential candidates to include into the plan as well. She may even consider girls who are not even love interests yet, like at the end of To-Love-Ru Darkness Chapter 32.5, when she wrote down the names of two of Mikan's friends, despite only having met Rito for the first time. She intends to include as many girls as she can because she feels that the more girls fall in love with Rito, the more difficult it would become for him to reject them all, and then he would more likely accept that Momo's harem plan would indeed be a logical solution.
As the series progresses, the plan continues to advance little by little. However, shortly after her arrival, Nemesis tells Momo to give up on the plan because she wants Rito all to herself as her servant, effectively making Nemesis the main antagonist against Momo's objective.