I've dabbled in writing a couple of detective stories before over at FIMFiction, and I like to play with the readers expectations in some of my other stories while showing a solid reasoning behind it, so I like to think that I know what I mean when I say that making an airtight detective story is both difficult and easy.
It starts with a goal in mind that stays strong as it leads itself backwards to the beginning of the case with branching possibilities along the way that are weaker in comparison. Let's take the first chapter for example, so...
!SPOILERS!
Three suspects: a wife, a son, and a mistress. The mistress usually have the weakest reason to kill their lover, and it plays no different here. The son has some bad ties with his father (it shows with the torn photo, stating that it was in the son's room in the evidences) and the idea of killing is there, but his father is trying to mend it (based off of the text messages). The wife... well what woman wouldn't want to kill her cheating husband? But she seems to show concern in the text messages as well (asking if he's home).
With poison at play, unfinished food present, and the lack of foresight on disposing the evidence, we can call this case closed. The son had unclear reasons at possibly wanting his father dead, but the wife holds the 'weapon' that was used in the soup. Yet there's an issue at present here...
There are 2 bowls that I'm seeing on the table.
This leads many into assuming that someone was with him, possibly eating from the 2nd bowl, and that's when the path to the culprit begins to crumble.
The wife placed poison in the pot of soup. She wasn't present based off of the text, so the victim must be eating with his son or mistress. It would be understandable that the wife wanted the mistress dead, but she's alive, so she couldn't have been eating the second soup. The son is alive too, but unlike the mistress, he hates his father. He said that he had to walk home, but that could've been a lie to cover his tracks. The son now seems to be the most likely suspect, because why would his mom try to poison him too?
1 pot, 2 bowls. The only two ways for the son to have escaped death was if he knew the soup was poisoned, or that he poisoned his father's soup. If the latter, how did the poison get in his mother's bag? He could've planted it, but he didn't really show any signs of him hating his mother. However, there is no other explanation.
Thinking up the murder is easy, but because of overlooking one small thing, the intended path of logic begins to collapse for another hypothesis to fill its place.
It's difficult to disguise the truth while still leaving solid clues to it.
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In other issues, there's a typo in chapter 0 when we examine the body.
"Victim has bruises all over 'her' face. 'He' had to be badly beaten."
I assume you mean "all over 'his' face."
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I'm giving this a three because it made me think for a while on the answer (even if that extra bowl misguided me) and that it feels complete. But because it feels complete, I'm feeling kinda bummed that there's really only one chapter. I know that this is considered a test for feedback and I hope you take my word and everyone else's words to heart, but after finally getting into the swing of things and then it's momentarily done, I'm left feeling... 'blueheaded'? 'Blueminded'? Whatever the term used for 'blueballed', but for the brain.
But in any case, I'm likely to change the stars the further progress you guys make.