Tegan and Turlough play chess while the Doctor tries to find the cause of the power loss which has struck the TARDIS. As he works, he hears a voice echoing in his head, and realizes that the power is being tapped by someone trying to communicate with him. Ordering Tegan to increase the power, he rushes out into the corridor, where the White Guardian appears, flickering fitfully. The White Guardian warns him of a danger to the balance of power and gives him a set of co-ordinates, but at that moment the Black Guardian materializes, announcing that all is for naught. As smoke pours out of the overheating TARDIS console, Turlough panics and reduces the power to prevent an explosion, and the Guardians vanish. The Doctor, furious, returns to the console room and sets the co-ordinates, failing to notice Turlough’s waxen expression when he tells Tegan he was given a warning by the White Guardian. The TARDIS materializes, and the Doctor takes Turlough out to investigate, telling Tegan to wait in the TARDIS in case the White Guardian tries to make contact again.

The TARDIS appears to have materialized in a warehouse, but the movement of the floor and a tar-covered rope indicate that they are in fact in the cargo hold of a sailing ship. The Doctor and Turlough hide when a ship’s officer enters the hold, and note his blank, staring expression -- almost as though he’s been hypnotised. When the coast is clear they emerge from the cargo hold and make their way to the crew’s quarters, where they learn that they appear to be in Edwardian England. The Doctor acts as though he’s just joined the crew, and is surprised to learn that they’ve been waiting for him to arrive -- until he remembers too late that in this era “Doctor” is the slang term for the ship’s cook. The crew are all rough but jolly sorts, definitely human, but they’ve been stuck below decks since boarding and none of them can remember their actual arrival on board. Most of them assume they’d been out drinking the night before, but Jackson doesn’t drink. All they know is that they’ve signed on to sail in some sort of race. The crew fall silent when an officer arrives to silently escort the Doctor away. There’s something odd about the officers on this ship...

The lights in the TARDIS begin to fail again, and Tegan increases the power and watches as the White Guardian flickers into existence. He warns her that the race must not be won -- winner takes all -- and at that point the console explodes and the Guardian vanishes. Tegan sees the ship’s first mate apparently peering in through the scanner screen at her, but he then topples and falls from the TARDIS, and she rushes out to help. When she emerges from the TARDIS, however, there is no sign of him outside. Another officer enters the hold, and when Tegan tries to hide she is caught by the first mate -- Marriner -- who seems to have suffered no ill effects from his apparent fall. He observes her with an odd, distant expression, and seems almost amused when he accuses her of stowing away. Tegan tries to flee through the dark, shadowy hold, only to find Marriner waiting at the top of the stairs. When he offers to take her to her friends, she assumes he’s already met them, for she hadn’t mentioned them yet.

The Doctor is irritated to see Tegan waiting for him in the officers’ mess, but listens as she passes on the warning. They’re obviously meant to stop the race in which this yacht is participating, but as yet don’t understand why it’s so significant to the Universe. When Captain Striker arrives they find him just as emotionally distant as his fellow officers. They sit down to dinner, but then the yacht yaws to one side and they realize that the wind has come up. The race is about to begin. After an odd toast to “an eventful passage and a stormy one”, the officers head for the wheelhouse. Marriner offers to escort Tegan and the Doctor sets off to collect Turlough before joining them.

As the yacht gets underway, Tegan begins to feel queasy, and Marriner takes her to a cabin which has been prepared for her. She’s too ill to notice how familiar it looks, but Marriner, clearly fascinated by her, offers her a mixture which will ease her illness. She falls asleep as Marriner tells her they’re about to round their first marker buoy.

The Doctor sends Turlough to ensure that Tegan is all right, but realizes that Striker has heard every word he and Turlough have whispered behind his back. Turlough locates Tegan, who is just waking to find that her stomach has now settled; presumably the drink Marriner gave her is the same drink they give the crew. Now that she’s recovered she realizes that her cabin is filled with familiar photographs, keepsakes and items of furniture, as if it was designed from her memories. As they return to the bridge Jackson contacts Turlough and gives him the stolen key to the rum ration. He’s convinced that the officers are slipping rum into the drinking water to calm down the crew, and urges Turlough to help him throw the rum locker overboard; without the drink to calm them, the crew will never climb the rigging. Jackson is dragged up on deck by the officers as Turlough tries to work out what to do.

Striker uses the viewscreen to highlight his competitors, and the Doctor confirms that the period detail is all correct except for a 17th-century Spanish ring worn by Critas the Greek. As they approach their first marker buoy, the planet Venus, the Doctor challenges Striker, demanding to know why he’s using a human crew. Striker refers to the humans as “Ephemerals” and realizes that the Doctor is a “Time Lord”, and the Doctor realizes that Striker is reading his mind. In response to the Doctor’s claim to be a lord of Time, Striker reveals that he and his fellow officers walk in the endless wastes of Eternity. Tegan and Turlough arrive in the wheelhouse just as the ship rounds Venus, and Striker cuts dangerously close to the atmosphere, running ahead of the solar wind on ion drive and using Venus’ gravity to slingshot further into the lead. Only Critas and the pirate ship Buccaneer come close to matching their lead -- until Critas’ galleon explodes. Striker and Marriner dismiss the explosion as the effect of Venus’ gravitational pull, but the Doctor believes that the galleon was sabotaged. Someone is willing to kill to win this race...

The Doctor sends Tegan and Turlough back to Tegan’s cabin to discuss the situation, hoping that the Eternals won’t be able to read their minds if they’re far enough away. The ever-present Marriner offers to escort Tegan back to her room, and Turlough takes the opportunity to try to contact the Black Guardian and find out what’s going on. But his Guardian, infuriated by Turlough’s inability to kill the Doctor, nearly throttles him to death and curses him to remain on this ship for the rest of his life. Tegan, berating Marriner for the deaths of Critas’ crew, learns to her further disgust that the Eternals on the ship were not killed but transferred back into Eternity while the humans aboard died. Marriner is fascinated by her sparkling mind, which is more alive than any Ephemeral he’s ever known.

The Doctor realizes that the Eternals are empty vessels, who rely upon Ephemeral minds for every idea they have -- and that while his mind is clouded with anger, for a brief moment Striker can’t read his thoughts. He meets Tegan at her cabin and plans to keep the Eternals occupied while trying to find a way to stop the race, but it’s all too much for Tegan to handle, especially Marriner. Although the Doctor is reluctant to give away the existence of the TARDIS, he agrees to let her sit this one out. On the way back they find Turlough unconscious on the floor, but when he recovers he claims simply to have fallen down. Terrified by the Black Guardian’s threat, he asks to wait in the TARDIS as well. But when they arrive in the hold, they find that the TARDIS is gone. The Doctor’s concern for his ship highlighted it in his thoughts; the Eternals saw it in his mind even while he was as far away as Tegan’s cabin.

An officer takes Tegan to Marriner, while the Doctor and Turlough are taken to Striker. Striker intends to allow the Doctor and Turlough on deck, but Turlough desperately tries to curry favour by betraying Jackson and handing over the key to the rum locker. Of course, Striker already knew, and was pleased with Jackson for providing further entertainment value. Jackson has since been brought back under their control, and Turlough has succeeded only in disappointing the Doctor. Before leaving the wheelhouse the Doctor asks what the purpose of the race is, and Striker tells him that the prize is Enlightenment, the wisdom which knows all and which will grant Striker his fondest desire.

By the time Jackson and the crew can fasten a life preserver to the ship and toss it out after Turlough, the boy is beyond reach. The officers refuse to turn back and risk losing the race for the sake of an Ephemeral, and Marriner casually tells Tegan that he won’t suffer long. Much to his surprise, however, the pirate ship Buccaneer turns aside, lowers a net and lifts Turlough to safety. The Doctor heads for the wheelhouse to try to convince Striker to let him visit the Buccaneer, while Marriner warns Tegan that Turlough would be better dead than with the captain of that ship. Realizing that Marriner is still reading her mind, Tegan angrily bars her thoughts to him, but this only fascinates him all the more.

Turlough is dragged to the Buccaneer’s wheelhouse, where he meets the voluptuous and gleefully sadistic Captain Wrack. Her first mate Mansell sets off with a jewelled sword intended as a gift for Captain Davey, and with invitations for the other ships’ captains. Wrack chains Turlough up and toys about his chest with a dagger, musing on how can entertain her. He tells her that he came to this ship because he knew she would win, and wants to know the secret of her power. Intrigued by the hidden depths of deviousness in his mind, she dismisses his manacles from existence.

Striker refuses to have anything to do with Wrack, telling the Doctor that Turlough has made his choice. But when Mansell arrives with an invitation to a reception on the Buccaneer, the Doctor accepts in Striker’s place, and Striker sees no reason to stop him. Marriner, learning that Tegan is going, offers to accompany them as escort and protection. Tegan changes into an elaborate gown in her cabin, but before they can leave, the yacht enters an asteroid storm. The Eternals won’t use their superior technology to evade collisions; that would be cheating, and this sort of excitement is all that makes their eternal lives bearable.

Mansell returns to the Buccaneer just as it enters the asteroid storm. Wrack learns that Captain Davey is catching up, and orders Mansell to hold level with him. She takes Turlough to a chamber in the hidden heart of the ship, but when he hesitates before entering, she closes the door in his face and locks him out. As he listens at the door, he hears a familiar voice inside... Striker’s ship successfully navigates the storm, but as they watch, Davey’s ship explodes moments before passing the Buccaneer. It would appear to have been hit by an asteroid, but the Doctor points out that the last ship to explode also did so while challenging Wrack...

The captains of the other ships in the race attend a reception in Wrack’s stateroom, and Turlough slips away to investigate the secret chamber alone. The Doctor, Tegan and Marriner arrive, and as Tegan mingles with Wrack’s other guests, Marriner searches for Turlough’s thoughts and locates him in the ion drive chamber. The Doctor sets off to collect him, telling Marriner to keep an eye on Tegan. But Mansell distracts Marriner, giving Wrack the chance to lead Tegan away from the others, claiming there’s someone in the wheelhouse she wants Tegan to meet. As soon as they’re alone, however, Wrack freezes Tegan in a moment of Time.

Turlough finds the ion drive chamber open to space, but he is protected by a vacuum shield -- until a passing pirate notices the open door, shuts it and turns off the shield. As the air in the chamber begins to empty into space Turlough tries to contact the Black Guardian, but the Guardian won’t lift a finger to save him. Desperate and terrified, Turlough begins to scream for the Doctor, who arrives just in time and switches the shield back on. As Turlough tries to pull himself together, the Doctor points out the eye shapes in the floor grid and the ceiling, and the crystal within the eye. Wrack may have used them to destroy the other ships, but she still needs something upon which to focus the energies. When Turlough tells him about the jewelled sword, the Doctor finally understands why Critas was wearing an out-of-period ring; it too must have been a gift from Wrack. Striker is too wary to accept a gift from her, but that won’t stop her from trying to smuggle a Focus aboard. The Doctor decides to stay on board the Buccaneer to stop Wrack from winning; she’s dangerous enough already, and with the power of Enlightenment she will be unstoppable. Of course, he must also find a way to stop all of the other ships in the race as well. But before he can decide what to do, he and Turlough are captured by Mansell and the other pirates...

Wrack awakens Tegan and takes her back to the stateroom, where Marriner is deeply concerned for her safety. She’s embarrassed by his fulsome claims that he is empty without her, and he’s surprised when she asks him if he’s in love. He doesn’t know what love is; he wants existence.

Turlough and the Doctor are brought before Wrack, and before the Doctor can say anything Turlough claims that he caught the Doctor spying and suggests that he, Tegan and Marriner be sent back to Striker’s ship. The Doctor has no choice but to play along, hoping that Turlough is just trying to prove himself. As they depart, Wrack forces her other guests to walk the plank. Since they are Eternal they cannot die, but once overboard they are out of the race. She then orders her crew to take Turlough to the plank, knowing that deep within his confused mind lies the desire to seize the prize for himself. As he is dragged out of the wheelhouse, he shouts that he heard the power that Wrack summoned in the grid room -- and that he serves that power, just as he will serve Wrack. Reading his mind, she knows that he speaks truth.

Back on Striker’s ship, Tegan changes into her ordinary clothing and joins the others in the wheelhouse. The crystal palace of the Enlighteners is now within sight, but the Buccaneer has hoisted her moonrakers and is pulling up, challenging Striker for first place. With great effort Striker manages to hold even, but the Doctor knows that Wrack can pass them at any time -- and suddenly realizes why she isn’t doing so. Somehow, she has smuggled a Focus onto the ship and is channelling the forces of darkness into it. It could be anything, no larger than a small gemstone -- and Tegan suddenly realizes what struck her as different about her tiara when she took it off.

As Mansell holds the ship level with Striker’s, Wrack takes Turlough to the ion chamber to see the source of her power. As Turlough watches in terror, the eye in the ceiling glows and a beam of dark energy envelops Wrack -- who begins to speak in the voice of the Black Guardian. The Doctor, Tegan and Marriner arrive in her cabin to see the jewel in the tiara glowing a bright, cherry red. The Doctor tries smashing the jewel to bits, but each fragment becomes a Focus on its own. He collects all of the pieces in his handkerchief, rushes for the deck, and with the last of his strength, flings the Focus overboard moments before it explodes. As the Doctor recovers from his exertion, Marriner informs him that he need only have imagined it going overboard; Marriner could then have converted that image to reality. The Doctor, irritated, realizes that the Eternals are far more dependent upon Ephemerals for fresh ideas than he’d previously thought.

Wrack awakens, furious that Striker’s ship has survived, but then the wind drops, leaving Striker becalmed. Since Wrack’s sails can catch even the barest whisper of a breeze, she must content herself with victory. Striker is unable to outrun Wrack, and Tegan doesn’t trust Turlough to stop her; Striker thus has no choice but to restore the Doctor’s TARDIS, which he reveals has been hidden inside the Doctor’s mind all this time. Marriner, however, refuses to let Tegan go without him, and as they’re running out of time, Tegan urges the Doctor to leave her behind. The Doctor takes the TARDIS to the ion drive chamber on the Buccaneer to try to talk some sense into Wrack, but instead of listening, she orders Turlough and Mansell to throw him overboard. Tegan, watching from Striker’s ship, is horrified to see two distant bodies fall overboard and vanish, as the Buccaneer crosses the finish line without even slowing. Marriner looks into Tegan’s mind at the moment of the Doctor’s death, and is confused by the lack of sparkle and grey shades he sees within. What is grief?

The Ephemeral crewmen are transported back to their own times as Striker, Marriner and Tegan cross to Wrack’s ship to pay homage to the winner. The Enlighteners -- the Black and White Guardians -- appear in the stateroom with the prize, and the Black Guardian crows over his victory. The Eternals have no understanding of good or evil, and when granted the ultimate power of Enlightenment, they will unleash chaos upon the Universe. But much to his surprise, the winner is not Wrack, but the Doctor, who brought the ship into port with Turlough’s help when Wrack and Mansell “fell” overboard. The Doctor turns down Enlightenment, claiming he isn’t ready for it. The White Guardian, satisfied, banishes Striker and Marriner back to the empty wastes of Eternity as Marriner begs Tegan to help him.