Murder at the museum

"Thirteen-year-old Agatha Oddlow's set to become the youngest member of the Gatekeepers' Guild, but before that, she's got a mystery to solve. There's been a murder at the British Museum and, although the police are investigating, Agatha suspects that they're missing a wider plot going on below London - a plot involving a disused Tube station, a huge fireworks display and five thousand tons of gold bullion"--Provided by publisher.