To fully understand the plot, you should have watched (Season 1) and the preceding episodes of Darkness 2nd . This OVA bridges gaps between major plot points in the "Darkness" arc involving Yami (Golden Darkness) and Nemesis.
This OVA (often labeled as Episode 2 or OVA 12 in the overall series) typically adapts chapters from the manga that were too explicit or tangential for the main TV broadcast. [Koten_Gars] To Love-ru Darkness 2nd - OVA 02 [...
Like most To Love-Ru OVAs, this episode contains high levels of fanservice (ecchi) and is uncensored compared to the TV version. To fully understand the plot, you should have