Josh and Jessica Murata are twin youngsters on a holiday.
Granny Murata is an agent with Team Obaasan.
Josh and Jessica do not know that fact about their paternal grandparent, of course, at least not at first. As far as they know, the beloved elderly woman who lives in Japan and not the UK as the twins do just is Granny; that and she owns a bookstore in Tokyo which the two avid readers find very exciting. Well, to be honest, Jessica is not that excited about reading - she rather be writing them - and Josh is far more interested in drawing comicbooks than in reading them but at least the bookshop has a lot of manga for him to study, "shelves and shelves of the Japanese comics".
To the twins, Granny Murata is a very traditional Japanese woman, always dressed as would be appropriate for someone of her age in a lilac kimono with print flowers on it. She is a tiny woman, even smaller perhaps than the twins. There was nothing in their minds that would make Granny anything different than what they knew of her, especially as they had already spent several summers with her over the years so they were certain then knew everything of importance there was to know. Then by sheer accident Josh discovered the special security console behind the large bookshelf and that would be the first indication of several that Granny might be living in "a luxury retirement home for the extremely paranoid". Push the right button, the twins would discover, and a door would open to allow them into an underground facility full of surprises. Starting with the fact that the place was the secret base for Team Obaasan.
Obaasan means 'Grandmother' in Japanese and Granny, as the twins think of her - much to her annoyance, insists on its use when they talk to her (they slip up all the time). Since we learn that Team Obaasan was a "group of crack crime-fighters who all turned out to be...geriatrics!", we can deduce the name was meant to apply to all of its members. Granny, it turns out, was their leader. She explains to the twins that "Team O is an elite, undercover, intelligence-gathering and crime-fighting unit of the National Police, operating under the Japanese Government. She adds that "The Sakura Apartment complex is our cover. Nobody will ever suspect a group of elderly people of doing the work we do."
Once the twins learn the truth about their Granny, they are determined that they must become a part of the team, albeit the junior auxiliary. While she is reluctant, she agrees on the condition that they go through a major process of training - all to make sure they are able to be worth of being one of the Super Ninja Spies.