A woman arrives in Guernsey, with her son Gilliat, and buys a house said to be haunted. The boy grows up, the woman dies. Gilliat becomes a good fisherman and sailor. People believe him to be a wizard.
In Guernsey also lives Mess Lethierry â a former sailor and owner of the first steamship of the island, the Durande â with his niece Deruchette. One day, near Christmas, when going to church, she sees Gilliat on the road behind her and writes his name in the snow. He sees this and becomes obsessed with her gesture. In time he falls in love with her and goes to play the bagpipes near her house.
Sieur Clubin, the trusted captain of Durande, sets up a plan to sink the ship on the Hanois reef and flee with a ship of Spanish smugglers, Tamaulipas. He gets in touch with Rantaine, a swindler who had stolen a large sum of money from Mess Lethierry many years ago. Clubin takes the money from Rantaine at gunpoint.
In thick fog, Clubin sails for the Hanois reef from where he can easily swim to the shore, meet the smugglers, and disappear, giving the appearance of having drowned. Because of the fog he has mistakenly arrived at the Douvres reef, which is still halfway between Guernsey and France. Left alone on the ship, he is terrified, but he sees a cutter and leaps into the water to catch it. At that moment he is grabbed by the leg and is pulled down to the bottom.
Everybody in Guernsey finds out about the shipwreck. Mess Lethierry is desperate to get the Durande's engine back. His niece declares she will marry the rescuer of the engine, and Mess Lethierry swears she will marry no other. Gilliat immediately takes up the mission, enduring hunger, thirst, and cold trying to free the engine from the wreck. In a battle with an octopus, he finds the skeleton of Clubin and the stolen money on the bottom of the sea.
Eventually he succeeds in returning the engine to Lethierry, who is very pleased and ready to honour his promise. Gilliat appears in front of the people as the rescuer but he declines to marry Deruchette because he had seen her accepting a marriage proposal made by Ebenezer Caudry, the young priest recently arrived on the island. He arranges their hurried wedding and helps them run away on the sailing ship Cashmere. In the end, with all his dreams shattered, Gilliat decides to wait for the tide sitting on the Gild Holm'Ur chair (a rock in the sea) and drowns as he watches the Cashmere disappear on the horizon.

Question: Who does Gilliat return the engine to?
Answer: Lethierry.
Alternative answer: Mess Lethierry