 The Thornberrys are in Africa along the banks of the Congo River. They have three days to catch a freighter at Matadi but if they miss it they will be stuck in Matadi for eight weeks. On the barge on the way to Matadi Eliza discovers that Kip and Biederman are also on board and they have a captured hippo with them which they plan to sell to a zoo. While helping the hippo escape Eliza, Darwin and Donnie fall overboard.