It was Remus/Sirius. It's title was something about "5 words". The five words were said to Lupin and made it so he knew that Sirius was innocent. I think they are a couple but on the outs because of all the suspicions. It is set after Hogwarts but before Azkaban.
I think at the beginning that it was a Full moon and Sirius showed up.
They were very unhappy with eachother. It is around "THE Halloween". After Sirius is arrested Remus tells Dumbledore about the "5 words" and convinces them that Sirius is innocent. I think Sirius spent one night in Azkaban.
Later they got custody of Harry and Sirius was very standoff-ish and filled with guilt for trusting Peter. Remus burns out caring for Harry and Sirius and that kinda knocks some sense into Sirius.
At the end there was another Full moon and it was completely different from the first one, though it started the same.
If you have any Ideas they would be much appreciated.