Plot

The Doctor, Yaz and Dan travel to 19th century China, where a small coastal village is under threat from both the fearsome pirate queen Madame Ching and a monstrous force which she unwittingly unleashes.

In a Chinese village in 1807, everyone runs inside and a woman enters the town and heads for the statue. A man, Ying Pai, goes out with a gun, warning his son that the woman has no idea what she'll unleash if she takes the statue. He tells his son, Ying Ki, that if anything happens to him then the duty passes to Ki.

The woman, Madame Ching, arrives at the statue of a Sea Devil holding a man, and she chips away at the base with her sword. The statue cracks, emitting a greenish light. Pai puts his sword to the woman's neck and tells her to stop, and she tells him to remove his sword. Pai warns that she no idea what she's doing, and she draws her sword as the statue shatters open, knocking them both done. The Sea Devil walks up to Pai and kills him with its own sword.

The TARDIS materializes on the village's shore and the Doctor leads Yaz out. She sonics the area and wonders why they're four centuries off. Dan comes out dressed as a pirate and says that he's wearing what Yaz gave him, and the Doctor says that he's dressed way too much. The Doctor clutches at her head as she hears a high-pitched whining noise, skims a stone out to sea, and determines from its disrupted path that there's a geomagnetic disturbance that might account for why the TARDIS was pulled off course.

The trio hear screaming and go to the village, and find Marissus killing people with its sword. The trio arrive and the Doctor tells Marissus to stop, and recognizes its species. It demands to know where "it" is, and demands to know who the Doctor is. She introduces herself and Yaz and Dan capture it in a net. Something enormous heads toward the village, its footsteps echoing in the air, and Marissus cuts itself free. A gigantic ship approaches the village, its underside glowing, and Marissus jumps aboard it. It's crewed by other Devils, and the ship makes out to sea,

Ching wakes up and sees the corpses around her, and Ki attacks her. She parries his sword and asks if he wants to die like Pai. He says that Pai was his father, and the Doctor and her companions over and the Doctor realizes that Pai was killed with a Devil poison. She tells Ki that Ching didn't kill his father, and Ching introduces herself. The Doctor recognizes Ching as the era's Pirate Queen, introduces herself, and sonics the area, and confirms the statue was recently broken. Ching says that it contained information she was seeking, the Lost Treasure of the Flor del la Mar. Legend says the treasure the treasure was found years after it disappeared years after it went down off the coast of Sumatra, and Ching explains that years late it was found by the great captain Sin Ji-Hun. Hi His ship went down in the area with the treasure still aboard. The Doctor confirms that there are pulse fields in the area, beyond the technology of the era, and wonders what the statue contain. Ki says that there was an ocean demon inside, and Ching released it.

Aboard the ship, Marissus says that he was betrayed, and entombed by the Land Crawlers. The call went out and the crew awoke, and now their time has returned. He tells them to let the Hua-Shen seek and feast.

The Hua-Shewn swims beneath the waters, and kills a fisherman.

Ki goes to the shore, and Dan follows him. Dan offers his condolences about Pai, and Ki says that they'll swim to Ching's ship and stop her.

The Doctor and Yaz to go to the TARDIS, and the Doctor complains that Yaz let Dan wander off. The Doctor explains that the Devils were on Earth before the humans, and she tries to get a trace on the Devil's ship. She discovers that they've masked their route to thwart her tracing them, and the Doctor figures that if they get the treasure, and they're leaving to find Ji-Hun by travelling through time, find the treasure, bring it back to bargain with Ching, and track down the Devils.

Dan and Ki swim out to Ching's ship, and find no sign of her crew. Ki wants to kill her to avenge his father, over Dan's objections, and figures Dan is 70. Ching finds them and holds a sword to Dan's neck, and figures that they're stowaways. She strings them upside down as stowaways, and tells them that she got what she came for and needs to get to the wreck of the ship. Ching says that she'll let them live if they help her pilot the ship to the lost reassure of Ji-Hun.

In 1533, the TARDIS materializes in the hold of Ji-Hun's ship and look above. They find Ji-Hun throwing the crew off his ship, and the Doctor sees a mist floating along the deck and realizes it isn't natural. A Devil appears from it, and the Doctor and Yaz hide from it. Ji-Hun kneels before it and tells it that his ship is it's. It asks if Ji-Hun has what they agreed, and the captain says that he does. The Hua-Shen growls in the distance, and the Devil says it won't honor an agreement with a "land crawler". It parries Ji-Hun's sword thrust. The ship starts to go down, and the Doctor and Yaz go back to the TARDIS and leave as the ship goes down. They figure they can go to the same spot the ship went down, in the future, and they find the lost treasure. They go out the doors, and are protected by an aquashield and an oxygen bubble.

The Doctor realizes that something is missing: the shipwreck. The Doctor confirms they're at the right coordinates, and the ocean floor collapses. The Hua-Shen emerges and bites the TARDIS, as the two women duck back inside and close the doors, and the creature draws back down into the abyss.

On Ching's ship, Ching tells the two men that none of their equipment works, and they'll have to go by celestial navigation. The stars in the sky movie, and Ching says that the world has been disrupted and nothing's more important than getting the treasure. When Dan wonders why it's so important, Ching shows them a severed ear in a box and tells them that her crew was taken, and they're being held hostage by Gao Podai and the black flag fleet. She has to pay a ransom to save them, and needs the treasure to do so and save her sons. Ching says that her sons are 3 and 6, and the longer she takes, the less likely she is to see them again. Something hits the ship from below, and then Hua-Shen approaches the ship. The trio man the cannon and fire, and the creature disappears beneath the waves. It spits the cannon balls into the air where they explode.

The Doctor and Yaz emerges from the TARDIS into the Devil ship, and Marissus Devil, Marissus, confronts them and says the TARDIs is his. The Doctor says that they were looking for Ji-Hun's ship, and they know Marissus causes Ji-Hun's ship to sink. The Doctor rambles on to give herself time to think, and figures that she's not the same as the other land crawlers. She tells it that she's met its species in the future, and wonders how it came to be inside the statue. It says that they made the ship their own, and leads the duo to a vast chamber containing the shipwreck. Marissus asks where the keystone is, and when the Doctor refuses to answer, tells her not to make the mistake that it wants to keep her alive. The Doctor asks it to show them the ship first, and Marissus takes them onto the shipwreck and says that the ship instills fear in land crawlers. Yaz asks what happened to Ji-Hun, and it takes them through the control room and shows them a force field cell holding Ji-Hun.

A signal goes off, and Marissus goes to check it. The Doctor asks what pact Ji-Hun made with the Devils, and he tells her that he was tricking them to save his men. He asks if the keystone is safe, and says that it's a tiny party of the treasure that his ship was carrying. It has infinite powers, and if someone holds the keystone, they can change the world: freeze life in time. transform matter. It's the last, vast power of a demon race, and the Doctor figures that it's Devil technology. Ji-Hun says that he gave it to his most trusted crewmember, and put him off his ship,

Marissus comes back, and the doctor claims that she has the keystone and Marissus will agree to her conditions before she returns it. they include releasing Ji-Hun. Marissus knows she doesn't have the keystone, and explains that the Hua-Shen has located it on the surface, and figures the Doctor is disposable. She jumps for the controls and ends the shipwreck to the surface.

Near Ching's ship, the Devil ship emerges from the water. The Doctor, Yaz, and Ji-Hun jump aboard Ching's ship, Ji-Hun wonders where the crew is, and when he realizes the trio is all there is, says that they don't stand a chance. The Doctor tells Ching that Marissus she released is looking for a keystone, and Ji-Hun says that it's a tiny orange jewel. A stone Ki has on a necklace glows, and Marissus appears in the mist, puts his sword to Ki's neck, and tells him to take off the keystone. Ki says that his mother got it from his grandfather and gave it to him, Lei Bo. Ji-Hun realizes that Lai Bo was one of his men, and must have made it to land and had a family.

Ki takes out the keystone, and Ji-Hun figures Marissus follow the keystone to land and was imprisoned by Lei Bo using the keystone. Marissus takes the keystone and teleports away in the mist, and the Doctor tells the others they can't let the Devil ship return beneath the waves. They jump aboard it and confront Marissus, and the Doctor figures that it's trying to flip Earth's geomagnetic poles, melt the ice, shift the current, change the pressure, and flood Earth. Marissus insists that they're claiming what was theirs, as their right, and it's done. The Doctor runs to the ship, and finds Marissus and its crewman men there. She tells the others to go, grabs a sword, and fence with Marissus. The crew fights the other Devils, and Dan and Yaz manage to defeat their opponent.

Ching parries the blow meant for Ki, and tells him that she should have saved her father's. She tells him to fight. Meanwhile, the Doctor is disarmed and tries to run from Marissus. The Doctor sonics his sword, disarming him, and runs for the sword. Marissus grabs her leg, and Ki kicks the sword to her. Ji-Hun kills Marissus, and the shifting of the poles starts.

As Yaz and Dan try to work the controls, Yaz wonders if the Doctor said something to Dan. He says that she didn't, and asks if Yaz can see it. Before Yaz can reply, the Doctor joins them, sonics the controls, and takes Ching's ship into the cavern. The Doctor tells Dan to keep the remaining Devils at bay, and gives him one of the Devils' swords before leaving with Yaz.

The Doctor leads Yaz to the Devils' control room. Meanwhile, Dan puts Ji-Hun in charge and he tells Ching to offload the treasure from his ship. She goes with Ki to go get it, while Dan kills the approaching Devils with one stroke.

The Doctor and Yaz work to shut down the reversal controls, and the Doctor figures she can internalize the charge, and keep any Devil from leaving. She admits that she doesn't go on dates very well, but if she did it would be with Yaz. She assures Yaz that Yaz is one of the greatest people she's ever known, including her wife, and will tell Yaz about her wife later. She tells Yaz that she can't be romantically attached to anyone because at some point time always runs out, then reduces the countdown from two minutes to 50 seconds. The Doctor has to hold the cable, and tells Yaz to get the others to the TARDIS. Ji-Hun arrives and says that they should go, and he'll hold the cable. He says that the world of 1807 isn't his, and they need to run. They run to the TARDIS, and the density increases trapping the Devil ship. Everyone gets to the TARDIs and the Doctor dematerializes it just in time.

The TARDIS materializes on Ching's ship, and they unload the treasure that Ching and Ki recovered. Ching figures it's enough to get her crew back, and wonders what she'll tell her crew about what happened. The Doctor advises Ching to tell them that she vanquished mythical creature to bring them home, and Ching says that Ki is her responsibility and she'll take him back to his village. She apologizes for what happened to Pai, and invites him to stay there as part of her crew. He hugs her and accepts, and she smiles and tells him never to do that again.

Ching asks the others who they are, and the Doctor says that they're just a crew with a ship, looking for adventure and often finding it. Yaz reminds the Doctor that she keeps promising them a trip to the beach, The Doctor takes them to a beach, and Dan calls Diane and starts to leave her a message about the mad adventures he's on and how he can't tell her about them. Diane calls him and says that she was just thinking about him, and she's been missing seeing him. She asks if he's going to be back anytime soon.

Yaz and the Doctor sit next to each other on the beach, and the Doctor tells Yaz that she can't fix herself to anything, anywhere, or anyone. She's never been able to, and that's what her live is. It's not because she want to, but if she fixes herself to someone, sooner or later it will hurt. Yaz tells the Doctor that her mother says that courage is doing something when you know it will hurt. The Doctor suggests they just live in the present, and Yaz agrees, then gives the Doctor a stone, and tells her to make a wish and skim it. The Doctor says that she wishes it would go on forever and skims the rock.