Sad animation I guess, I suppose it's similar to "I want to eat your Pancreas" and "Your Lie in April". Basically, the protagonist, from the beginning, already had lost his memories. Each episode, he gains back each piece, then near the end, he pieces them all together, finds out the truth about the past and falls into a deep trauma, and his past haunts him.
He forgets 2 important people in this story, and he remembers in the end. But then we get a cliff hanger and that sucks, we never know what will happen.
The marble that the protagonist has, it's the key that connects the past, present and future, or maybe it's the way I see it. The marble and the picture, were the start of everything in this animation, and you will see it after you watch it.
It's very confusing, the story, so basically, he loses his memories of the past and gains them back in the end. But, there will always be friends that try to keep you away from recalling your memories, and those people exist here, his 2 classmates, they kept making him forget.
I do recommend this animation, it's not particularly bad, but I know some people may dislike the Chinese voice-over in this animation. It's still worth a try to watch it.
I'll also mark this as a spoiler, well, I think spoilers are for "future episodes" or "manga/novel/toon/manhwa spoilers", but I'm just making some references, so I hope it's not too much of a spoiler.
Though, there is an OVA, I'm not sure if that is connected or the ending, but I'll go watch it now.