Millionaire Detective may not always have been up on its mystery tropes, but in the end it fell back on one that's been a standard since Alexandre Dumas' 1854 novel The Mohicans of Paris: cherchez la femme. The term, which simply translates to “look for the woman,” has some fairly sexist origins, which aren't entirely eradicated by its use as a trope of detective fiction, where it implies that at the root of the crime there simply must be a woman, typically assumed to be a femme fatale. In the case of this particular series, the woman in question isn't sexy cousin Suzue, but rather Daisuke's adorable grandmother, who, it turns out, has been pulling the strings all along.
In some ways that's a nice twist – after all, who would ever suspect a grandma of being a criminal mastermind? It also says something about the sexist nature of the story's world (which, lest we forget, originates in a novel from the 1970s) that for all of the times in the second half of the show that someone referred to “the boss” or “the head”, everyone assumed that the person in question must be a man – a fact that the show was perfectly happy to play up with the assumed return of Daisuke's missing father. Granny covered her tracks very well, and the loyalty she commanded from her henchmen was truly impressive; in fact, that loyalty and the expectation of it could be said to be her ultimate downfall, as she never expected Daisuke and Suzue to go against her wishes. And you know, I'm not sure that they could have, had not an unwitting Haru taken the decision out of their hands. Even more than the night at his apartment, mistakenly releasing the details of adollium into the world may be the kindest thing Haru ever did for Daisuke, because it forced him to go against the woman who had more to do with his upbringing than his mother did (because she arranged for her murder, but whatever), something he may not have been able to do on his own. And once that was done, slapping the cuffs on her must have felt like the easy part.
While elderly sleuths like Miss Marple and Jessica Fletcher certainly aren't in short supply, elderly villains, especially lady villains, are much less common, so this was quite a bold move on the show's part. (And if you want to read about an even more charming old villain, check out the diabolical Maud in Helene Thurston's An Elderly Lady is up to No Good.) That Shigemaru was simply being impersonated by a guy in a Scooby Doo-style mask is a bit of a let down – especially since the sad truth is that Shigemaru is in some sort of long-term care facility – but the core motive is one that should perhaps make Daisuke think about what he does with the rest of his life. That motive is one of the classics: money, and it was the profit from adollium and its secret sales that in large part gave Daisuke the life that he's been living.
So how will he reckon with this? It is, essentially, blood money, and it's paid for HEUSC, the clothes, the cars, the gadgets…you name it. It's easy to say that Daisuke and Suzue should just give it all up because that's the right thing to do, but that would be far-fetched even for a show with the gadgetry this one has. Instead Daisuke's solution seems to be to use the money to do good – he and Haru are using his tech to track adollium peddlers all over the world while HEUSC now works for the Metropolitan Police. It's not a perfect compromise, but it is one that seems to be working at story's end.
Haru has put his past trauma behind him. He doesn't need to be able to shoot a gun to be a good cop, and his new gear allows him to really lean into that. While his and Daisuke's dynamic hasn't really changed, they are now comfortable with themselves and each other, and that seems about right. Daisuke's still a jerk – a role he seems to have settled into when he gained access to his family's tech from the flashback we get – but at least he's put his personal monster to bed as well. That leaves us with an ending that's conclusive enough while still letting the characters live on, and I feel like that fits the story well, because that's basically how the whole thing has been: good enough and no more than that. If we got to pick an okay-est show of the season, this would be my choice.