Set sail to Atlantis with Tim and his friends in the laugh-out-loud funny, highly illustrated fourth book of the New York Times bestselling Once Upon a Tim series from Spy School author Stuart Gibbs. Tim and his ragtag crew have been rescued from certain doom by a mermaid princess--but now, she wants something in return for saving their lives: her father King Neptuna's stolen trident. To pay their debt, Tim, Belinda, Ferkle, Rover, and Princess Grace once again brave the Sea of Terror to track down the trident in the glorious city of Atlantis, which hasn't sunk into the sea yet. (In fact, it is famed as the safest city on earth.) But there is plenty of danger en route--and the notorious Prince Ruprecht is lurking about as well. Can the junior knights evade the scheming royal and complete their quest?