I've finished the first case in this game so far – at first I was planning on reading some Sherlock Holmes before this but I'm glad I didn't because reading the stories the cases are based on just gives away the ending. They do add in some elements that aren't in the original stories and change some things so it could still be interesting for those who have read them but it still spoils the most important clues and conclusions.
Since I read the story the first case was based on after I finished it, I know I picked the right conclusion but I still have some questions ... maybe someone else who has played it can help me:
EDIT: After reading what's below see my comment also because I realized I'm still overlooking some important things lol.
I was really stuck at first between the killer being Nelligan and the sailor because I couldn't figure out how the notebook had gotten there if Nelligan hadn't been there during or after the murder. I figured there were two scenarios – the sailor had gone to Black Peter's before he was killed and then Nelligan came and killed him after he left, or, the sailor had found Nelligan's notebook at the hotel (where he said he left it during the interview) and killed Black Peter and that's how it got there. The first one didn't make sense to me because if the sailor had gone to the cabin, why wouldn't he have mentioned that in the conversation with Holmes, and why would he just leave without taking anything or doing anything to Black Peter when he hated him so much? It also made more sense that a strong harpooner could kill him rather than Nelligan with a lucky throw. But I was still so stuck on the notebook. Ultimately I decided the sailor must have gotten his hands on the notebook and that's how it got there.
After I read the story I realized how the notebook could be explained. In the actual story, it explains that Nelligan went there the night Black Peter was murdered but was so shocked at seeing Black Peter dead that he dropped the notebook and left. That explains the thing that was bothering me so much, but I can't remember if that was mentioned at all in the interview with Nelligan. I'll have to see if it's possible to go back and check that because that would have been an oversight by me and would have made my conclusion easier if I had had that in mind. If he never mentioned being at the crime scene after the murder had already happened, it does make the case trickier to figure out.
The other confusion I have is about the tin box, and the timeline of when/if the securities were in there and when the gardener took it and put the letters in there. The most logical conclusion I can think of is that the book with the securities in it was inside the tin box, so the gardener (not realizing the book was important) took the book out and put it on the shelf, then the sailor found them in the book and took them. Does that make sense? That would make the timeline – gardener goes in the cabin first when no one's there and takes the box, puts the securities on the shelf; sailor goes there when Black Peter is there, kills Peter, takes the securities; Nelligan goes there after Peter is dead, drops his notebook, leaves without taking anything. What I don't get is why the gardener would specifically choose that box to put the letters in when he could have put them in anything. Wouldn't that just raise more suspicion with Peter if the box is missing? Let me know if you have any better theories about how the tin box plays into this and if I'm misremembering anything or forgetting anything.
As frustrated as I might be with the case not being as airtight at the end as I would have liked it to be, it's possible there are just key things I overlooked that would have made it come together more cleanly. I have some other gripes with the game, as well as compliments, but I'll save them for my actual review.