The Case of the Vanishing Ventriloquist by E.W. Hildick, 1985.

Mari Yoshimura, Wanda’s pen pal from Osaka, Japan has just arrived in the United States, and she’s eager to meet Wanda’s friends. Mari’s father is the head of Yoshimura Electronics, and he is visiting different cities in the United States on business. While her father travels, Mari gets to enjoy an extended visit with Wanda. Wanda has told her all about the McGurk Organization, and Mari is eager to join up with them during her stay in America. Unfortunately, when she first arrives, McGurk isn’t in a very receptive mood.

McGurk tells Mari that she can’t join the organization, which hurts Mari and offends Wanda, because he has organized a series of challenges in order to decide which of the current members to give a promotion. McGurk thinks that Mari’s presence would upset the challenges, and he can’t promote her because she hasn’t actually done anything with the organization yet. However, Wanda negotiates with McGurk. Since Mari is her guest, and she can’t neglect her guest, she arranges for Mari to just follow along on the challenges, working through them herself just for fun. McGurk allows it on the condition that Mari not help Wanda because that would give Wanda an unfair advantage. Wanda and Mari agree to the arrangement, and Mari writes all of her notes for the challenges in Japanese, just to make sure that Wanda doesn’t accidentally see any of her answers.

Mari turns out to be really good at the challenges that McGurk sets. When he tells the members of the organization to spend a day observing people and notice how many times people do things that would be a temptation to criminals (like leaving packages in a car, tempting someone to break in and get them), Mari ends up with more observations than anyone else. Mari also proves to be good at noticing suspicious behavior when she sees a man that no one else notices, who seems to be hanging around a bus stop for no reason, not showing any interest in getting on any of the buses.

Then, Brains accidentally discovers a real mystery that the McGurk Organization can investigate where Mari plays a special role. While Brains is working on one of his latest inventions, a new kind of portable phone for kids (this is before cell phones became popular), he accidentally gets his signals crossed and ends up overhearing part of someone else’s conversation. It sounds like the two men Brains overhears are going to target someone at the Senior Citizens’ Annual Picnic. However, because Brains didn’t hear the whole conversation, they can’t be sure what these men are going to do. They report the incident to Patrolman Cassidy at the police station, but he doesn’t think too much of it. He says that he’ll look in on the picnic but that what Brains overheard might not really have to do with a crime. He heard too little of the conversation to be sure what the men were actually talking about.

Fortunately, because Wanda’s mother is part of the committee organizing the picnic, the kids have a good opportunity to investigate the matter themselves. Wanda will be helping her mother to serve food, and Mari is going to be part of the entertainment, putting on her ventriloquist act. Mari says that the other members of the organization can be part of her act, so they can be on hand to keep an eye on things. McGurk is pleased about this and finally offers Mari a position as a trainee of the McGurk Organization.

However, it turns out that everyone has completely misjudged the situation. A very serious crime is being planned, and the McGurk Organization doesn’t realize it until Mari is kidnapped from the picnic! Mari was the target all along, and the suspicious man at the bus stop was actually there to watch her. Can the others get her back before it’s too late?

From this book on, Mari becomes a regular character in the series and a full member of the McGurk Organization. Mari’s father decides that he wants to open one of his electronics factories in the United States, so Mari and her family will be living there for awhile to oversee it, giving Mari the chance to stay with the McGurk Organization for an extended period of time.  McGurk starts dreaming that when Mari eventually goes back to Japan, she will open a branch of the McGurk Organization there, but that would be years in the future, if it happens.  McGurk dreams big.

One of the funniest parts of this book is when the kids are supposed to be looking around for examples of suspicious behavior. Before the challenge begins, McGurk admits that what is “suspicious” is difficult to quantify and that most of what they’ll notice will have perfectly reasonable, non-criminal explanations behind it. Joey Rockaway notes that, for most of that particular challenge, the members of the McGurk Organization themselves are the ones who are acting most suspiciously, running around and spying on random people. At one point, Joey almost gets thrown out of a supermarket because the manager noticed the creepy way he kept spying on a woman who kept picking up packages of cookies and then putting them back. It turns out that the manager of the store knows that the woman is on a diet and has had trouble wrestling with temptation. She routinely gets tempted to buy cookies, picks some up, and then puts them back on the shelf when she realizes that she shouldn’t have them. Her behavior may look odd to people who don’t understand what she’s going through or what she’s doing, but perfectly understandable to those who do, like so many things.

The book is currently available online through Internet Archive.