This book is about three kids who spend the day aboard a harbor tug named Pegasus. They get to meet Captain Susan Smith and the other important people on board. Captain Smith drives the tugboat from the pilothouse. Ed is the engineer who takes care of the engine. Tim and John are the strong deckhands who handle all the ropes. Maria is the hardworking cook who keeps everyone fed. The kids learn about all the different parts of a tugboat. They learn about navigation. They visit the engine room and see a diesel engine. They learn how a tugboat handles ships much bigger than itself. In this busy day on the water, Pegasus helps dock a container ship. It prevents an oil spill from a sinking tanker. It pushes a barge up a long and winding river. Finally, Pegasus helps a cruise ship start its trip to The Bahamas.