When eleven-year-old Finn is given a fish tank by a complete stranger in the street it is the start of an amazing adventure. That evening, he sets it up at home. In the middle of the night, he is woken up by a crab making noises in the tank. When he moves towards the tank, he discovers an octopus sitting in the corner, who then pulls him into the tank. There he discovers a fantastic underwater world called Adria, inhabited by miniature humans called Aquaplings. One of them is a famous inventor called Ged Sinclair. Finn joins Ged on a mission to rescue his missing son, Henry.
Whilst there, Finn becomes entangled in long-standing tensions and a battle-in-the-making between The Rotterbogs, a community of water ogres, who want to regain control of Adria and ordinary Aquaplings. It is only later he learns that his being there was no chance encounter; he was in fact placed there by the Rotterbogs to be used as a pawn to obtain information from Ged, to assist in the battle.
As he deals with the challenges presented by The Rotterbogs and the water landscape, Finn evolves from a quiet, curious boy into a confident and courageous one.
