On this page of our guide to Call of the Sea, you will find the solution to the puzzle found in the prologue - A Family Heirloom. We described how to open the suitcase and where to get the code.
The suitcase is on the desk in the right corner of the cabin. This is the only thing you have to open in this chapter. If you don't do that, you won't be able to leave. In the suitcase, you will find a key required to make progress in the plot.
The suitcase is locked with a three-digit code. The heroine will immediately mention that she always forgets it. However, she is sure that she has written it down somewhere.
Check out the journal - it has the code. On the first page, you'll see a picture that says 506.
After finding the code, you need to change the numbers on the suitcase's lock, and then press the last button. This will open the case and let you check out the contents.