Bully and Monica are victims of an ancient curse. Each full moon one of them is turned into an animal (Monica to a hawk, Bully to a fish) and the other is reverts to human. Their only chance to break the curse is to reach the Temple of the Sun in the sunken city of Hidrionis in time for the next solar eclipse.
Like other Opera Soft games, Sol Negra is a side-scrolling action game split in two parts, the second part being accessible with a code given at the end of the first one. The first part has the player as Bully try to locate the key to the cage holding Monica in her hawk form. Once she's freed, the two must travel to the entrance of Hidrionis. The second part takes place underwater, as Monica tries to reach the entrance of the Temple while protecting Bully in fish form.