On a trip to Port Mobius Sonic, Manic and Sonia are told that ships are sinking due to the Mobian sea-beast Moby Deep. The hedgehogs also meet Captain Squeege who informs them of a scam in which the townspeople must rely on Robotnik for protection from the sea monster. In reality it is Robotnik who is sinking the ships. Sonic must overcome his fear of water and the monster to find the truth and end the fears of the seaside town.