WARNING: A FEW SPOILERS
I was introduced to this show when I watched it dubbed in my home country during a summer visit, back when I was around 9/10 years old. I fell in love with it and began to look it up online, and it wasn't long before I began watching it subbed. The story's premise might seem strange and maybe silly to some, but the author uses it to its full potential to create a brilliant anime/manga. Basically, the main character, Shinichi Kudo, is a famous high school detective who uses his geniius to slove difficult murder cases. One day when he is visiting an amusement park with his childhood friend, Mouri Ran, he runs and is shrunk after he witnesses a suspicious transaction between a man dressed in black and a business man. He gets knocked out from behind by the man's partner and is given a pill that's supposed to kill him, and he is left for dad. When he awakens, though, he finds himself shrunk to an elementary school kid. He renames himself Conan and sets off on a quest to get his body back.
Plot: DC actually has little plot for around the first 127 episodes. Sounds crazy, right? Of course, that's only if you consider Black Organization (BO)- related stuff to be the only plot. In the first 127 episodes stuff does happen, characters are introduced, and a LOT does take place. Its just that the organization that shrunk Shinichi into Conan barely appears. Once you reach episode 128, though, the organization does begin to show up a lot more, and the story really kicks off once Haibara Ai, one of the top scientists of the BO, the one who made the drug that shrunk Shinichi, appears and confronts him. From there on the plot becomes a real thrill, and is full of suspense, action, mysterious characters, complex mysteries, and mind-blowing plot twists. There are usually long waits between episodes where the plot moves forward, but if you're patient you'll be rewarded with a story you won't forget anytime soon.
Also, don't be fooled by the cheerful picture above; Detective Conan is definitely lighthearted at times, bu it can also become petty dark rather quickly. There are some tragic murder cases that often deal with very mature themes that children shouldn't be exposed to, and things get dead serious when the Black Organization gets involved. The more you learn about them, the more you realize that they are not people you mess around with.
Characters: The characters in this series are lovable and likable, and there's an abundance to love an like. The main cast is very well developed; Shinichi changes a lot as Conan, and his character development is fascinating as he changes thanks to the experiences he goes through which mature him. Haibara Ai has some wonderful development to as she starts off as a cynical, suicidal person who sees only the dark side of human nature, and actually makes some profound observations on it, but slowly changes thanks to Conan and the Detective Boy's influence. There are too many characters to mention here, but I'll just say the author does an excellent job writing their different personalities. There are a lot of characters to love, both the deep ones and the fun ones.
Mystery: So yeah, this is basically to the show what action is to action shows, and let me say that the mystery cases in this show are MIND-BLOWINGLY INCREDIBLE. I can't emphasize this enough, the author is a GENIUS. The mysteries are nothing like Scooby-Doo mysteries; instead they are complex, intricate, and often difficult to solve. There will be many where you will be utterly stunned by the cleverness of the solution. Expect tons of mysteries (literally nearly every episode has a mystery, after all it is a mystery show) which you are likely to enjoy. The murder cases are often so good you can go back and rewatch the case even if you know the conclusion. Seriously, the mysteries are great. Also, if you like gore, there are plenty of bloody and brutal murders to satisfy you, although they decrease in number a bit as the series the beginning the murder cases start out a bit simple, yet still clever, but they gradually grow more complex as the series goes on, with some of the later cases being so complex you can literally barely make heads or toes of it. I've seen some people comment that this anime has murder cases superior to the ones you see in American tv shows, so you get the idea.
Music: Awesome, the soundtrack fits the mystery aspect perfectly, and there is always a brilliant track for every occasion, whether it is romantic, funny, mysterious, or serious.
Animation: Since the show is so incredibly old, the animation in the first episodes is not the greatest, and generally, because the series is so long and there isn't much action to animate, its made on an ok budget. The animation does improve substantially the the later episodes, though.
Well, I know all that wasn't even close to covering the whole show. It's simply too huge (nearly 700 episodes at the time of this writing) to cover properly in such a short review, so alas, I have definitely not done it justice. It's my favorite anime, and I HIGHLY recommend it to anyone, regardless of gender or interests. One of the many wonderful aspects of this show is its broad appeal, as it literally contains almost everything: mystery, thrill, suspense, comedy, romance, action and more. Even if you're not a mystery fan you're likely to still love this show. Its length might discourage you, but trust me, its completely worth it, or else you'll be missing out on an experience you won'tr get anywhere else.
This anime's awesome.