The year is 1906, the place is San Francisco. Annie and her brother, Jack, have just traveled here in their magic tree house, on a mission from Morgan le Fay, the mysterious magical librarian from King Arthur's time. In an effort to save Camelot, the children have already found three special kinds of writing for Morgan's library: something to follow (Civil War on Sunday), something to send (Revolutionary War on Wednesday), and something to learn (Twister on Tuesday). Now it's time to find "something to lend." It's a quiet, peaceful morning in San Francisco, and Annie is eager to start exploring. So eager, in fact, that she pulls Jack away from his research just before he would have learned a very important piece of information... All too soon, the siblings figure it out for themselves: they have arrived in this lovely city a moment before one of the biggest earthquakes the U.S. has ever known shakes the Bay Area to pieces! Stunned, Jack and Annie wander the streets, but quickly find a purpose. Lots of people need help transporting goods to safety, and many more are left without any idea where to go or what to do. But what about their mission? Will the kids find something to lend before the entire city goes up in flames?