Okay, what? I thought I saw To Love-Ru Darkness OVA episode 1 about a month ago? There must be a miss-label or something.
So this series basically follows the story just like the manga. If I'm not mistaken, things will get really dark and echii since I read the manga. But the nice thing is about this series, it focuses on some heroines named Momo, Golden Darkness(the assassin) and along with a new character which the OP is probably spoiling to you. Also, they might focus on the action just like the manga itself. Though the unfortunate thing about this series is that it's an "OVA" type.
Other than that, the series re-introduces what the main story left off and it isn't that bad. To wrap things up, this anime is definitely on my watch since I watched the previous seasons and I like echii.
Conclusion: Definitely on my watch list since I watched the previous seasons...