One morning, two years after Petey Myshkin Stephenson's mysterious adventures in "Otherwise," he and his best friend Chace Fusillade, who is very skeptical about this place "that both does-and doesn't-exist," are both swept away to sea and abducted by pirates in a world that has hardly changed in two and a half centuries, and where there is no U.S.A. because the original colonies lost the war against the British. There the boys discover they have been locked in together with a beautiful Chantilly cat and a bulldog, who they soon befriend.
After a sea battle with two enemy ships, the ship is wrecked, and the boys become castaways with the two animals.
A storm sweeps Petey and Chace to an island in a place called Biestia ruled by animals. The island is ruled by foxes, who imprison the boys. Word that these two denizens of humanity have been captured rapidly spreads, and representatives of all of the animals of Biestia converge on the island to try the boys for the crimes of humanity against the animal kingdom.
Miraculously, the Chantilly cat and the bulldog show up at the trial (they had been swept up by the storm to another part of the island) and defend the boys before the animal court, which erupts in fury.
The four escape and are hidden in a cave they have to burrow through after many a misadventure to escape. After climbing across a mountain in the middle of the island in flight from their pursuers, they discover, anchored in a harbor on the island, the two "enemy" ships that had sunk the pirate ship; they are actually crewed by cats and dogs related to their friends.
The Chantilly and the bulldog (Madame Tontine and Hamilcar by name) are both best friends and members of the royal houses of their respective kingdoms; they had been abducted by the pirates long ago and kept for ransom. There is jubilation by the two crews (one of which is manned by the Chantilly's daughter) now that their search is ended.
Another sea battle is fought as the two ships, discovered by the enraged beasts of Biestia, try to escape. The dogs' ship sinks in the battle, and the cats' ship escapes out to sea. But the bulldog manages to cling to piece of wreckage and is later rescued.
The four companions return to the land they all had started from - geographically the same, though so different given their different "histories" - to great rejoicing among the dogs and cats, and there is a pledge of mutual allegiance between Petey and Chace and the