For anybody who hasn’t read the first book:
Page 12 references the 消えたデメキン事件 (Case of the Vanishing Goldfish), which was one of the cases in the first book. Spitz helped out Inspector Kuroboshi with the case. I’ll flag up if there’s any further context you need, but that’s probably it!
Also, I’m going to take this opportunity to flag what I like to call the “Never trust Japanese commas” rule
On page 10, you have the sentence 犬たちは しかたなく、おれがおしえた、うでたてふせをはじめた。
That past tense verb in the middle might throw you if you haven’t encountered relative clauses in Japanese before. These are a SUPER COMMON construction. They’re basically equivalent to - in English - saying something like “the bus which I caught to town”, or “the person who lived next door”. In other words a little descriptive clause which modifies (describes) a noun.
The main difference in Japanese is that they precede the noun they are modifying, instead of following it. The relative clause modifying the noun will be in plain / dictionary form.
In this case, we have the noun うでたてふせ, being modified by おれがおしえた. That’s the agent おれ and the past tense of the verb おしえる, “to teach”. So it’s “the press-up I’d taught them”.
Then you just treat that little clause like its own thing. They get はじめる’d by the 犬たち.
Also, wth Spitz?! What kind of a trick is that to teach a dog?!
The comma in the middle of the relative clause - between おれがおしえた and the noun it’s modifying - could throw you, because it looks like it breaks the sentence in two there. But remember: never trust a Japanese comma! Their placement follows no human law.