The Courageous Warriors of the Garden of Hope have summoned Nagisa, Honoka, and Hikari to protect their kingdom from a witch who plans to steal the Diamond Line, which is a set of jewelry the queen of the Garden of Hope wears to a ceremony every year to replenish the water of the kingdom. The witch plans to resurrect the Dark Lord and if she does so, not only will the Garden of Hope be destroyed, but other kingdoms in the galaxies, including the Garden of Rainbows and the Garden of Light.
(Source: Anime News Network)
— Description from AniList community (anilist.co).
