The Thimble Theatre storyline known as "Dice Island" or "Bernice the Whiffle Hen" (published between 1928 and 1929) is perhaps the most important in the strip, as it introduces the character who would become its undisputed star, and a popular culture mainstay: Popeye the Sailor. 'Dice Island' is also the name of the locale where much of the action takes place.
Castor Oyl obtains Bernice, an 'African escape hen', from his uncle Lubry Kent Oyl, who raised her from the egg as, he explains, such a bird cannot be caught, and proceeds to showcase her escaping abilities. Uncle Lubry goes on to offer Castor one thousand dollars if he is able to kill the hen. Castor fails at every attempt, even as Bernice grows attached to her new master and does not leave him alone, staying with Castor after Uncle Lubry leaves. Certain individuals become interested in Bernice, to Castor's chagrin, most notably a mysterious hooded woman. She is revealed to work for the unscrupulous millionaire Fadewell, and Castor is taken to the latter's house along with his "pet". Fadewell is overheard saying that Bernice, a Whiffle Hen, is able to grant good luck to whomever rubs her head's feathers. He owns a great casino on the faraway Dice Island and wants her to aid his operation, so he buys her from Castor. Castor however is compelled to remain at the mansion as Bernice will not separate from him. Yet, he takes the first chance to evade his guard and run away, with Bernice in tow, planning to buy a ship with the money he got from Fadewell and travel to Dice Island along with his friend Ham Gravy in order to partake on the casino's millions thanks to the bird's luck.
Castor and Ham buy a sailing ship and hire Popeye, a crusty sailor they meet at the docks. He soon finds he is the ship's only crewman, and is instructed to "tidy up the ship, batten down the hatches, scrub the decks, pull up the anchor, get three sheets to the wind and then go to helm". After they sail off, Popeye is bothered to find a stowaway: Castor's sister Olive Oyl, who wanted to tag along for the trip and is reluctantly allowed to stay on board. Captain Castor and Popeye frequently argue, with the former often abusing his authority. Castor discovers the sailor's weakness for dice gambling and, thanks to Bernice's powers, uses it to win back the wages he pays him. At first unconvinced, Popeye also takes to rubbing the hen's feathers and wins some money back-- allowing him to buy himself a new black shirt that would become his trademark.
Eventually, they reach the shores of Dice Island, which are infested by formerly rich men turned beggars. Castor begins his winning streak at the casino, running afoul of manager John Stork, while Popeye and the rest are to catch the money bags Castor throws out the window. With enough cash won, they decide to leave just as the casino's owner arrives. Only Fadewell's underling, John Stork is ordered to make up for the losses by following the departing ship on a small boat, coming aboard and earning the group's sympathies then betraying them. Olive, Castor and Ham are easily charmed, only Popeye distrusts the "disgraced" Stork, seizing every chance to sock him. When the time is right, Stork produces a gun and prepares to shoot the meddling sailor. Popeye punches away at his enemy even as he is shot a total fifteen times. The sailor seemingly finished, Stork turns his sinister attention to the rest, however, the fallen Popeye has been rubbing Bernice's head for her power and is miraculously able to stand again and knock Stork out with one last mighty punch even as he takes a sixteenth shot. Then, Fadewell also boards the ship but both villains end up knocked out and thrown back to their yatch. For a while, it seems as though the wounded Popeye is going to die, but he keeps turning to Bernice for all the luck he can get, and eventually recovers. Weeks later, the Oyl siblings' parents Cole and Nana Oyl welcome back the travelers and are presented with multiple money bags.