Between the striations of the timelines, in a theoretical no-where no-time, a slaver ship is trapped with a cargo of Tharils on board; time-sensitive creatures like human lions, capable of seeing between the timelines and navigating time-travelling ships such as this one. But the navigator of the slaver ship, Biroc, is refusing to visualise, and the frustrated Captain Rorvik tries to force him to do so by launching the ship blind. The ship thus flies directly into a time rift and is seriously damaged. As the crew struggles to put down again, Biroc catches a glimpse of the TARDIS... The crew manages to put down back in the same void where they've been stuck for so long; but now their outer hull has been breached and the warp motors are shot. Biroc apparently passes out from the pain, and Rorvik orders two of his crewmen to take him to be patched up. But Biroc recovers and escapes, phasing into a different timeline as he runs out of the ship and flees through the void.

The TARDIS has also been caught in the time rift, and as the Doctor and Romana argue over what to do about it, Romana facetiously suggests tossing a coin. The Doctor wonders aloud why that wouldn't work, citing the I Ching and the holistic nature of the Universe. Unfortunately, Adric takes him seriously, flips a coin and operates a random control on the TARDIS console -- and the doors open, flooding the console room with Time Winds. The Doctor's hand is caught in the Time Winds and ages rapidly, and K9 is seriously damaged as the Time Winds scour through him. But then Biroc enters, out of phase on a different timeline, and navigates the TARDIS to a safe landing. Biroc tells the Doctor and his companions that he is a shadow of his own past -- and of their future. Others will be following him; they are not be trusted. Biroc departs, and the Doctor, realizing that the co-ordinates on the console are all set at zero, follows him into the void, hoping to find some answers. Romana and Adric realize that if E-Space carries negative co-ordinates and N-Space positive, then they must be at, or near, the intersection point...

Rorvik is infuriated by his crew's apathy, as they seem to have given up all hope of escaping. Sagan picks up a reading from the void, indicating the presence of the TARDIS, and Rorvik decides to investigate with Lane and Packard. The work-shirking Aldo and Royce reluctantly break the portable mass detector out of stores, and Rorvik's expedition sets off, using the mass detector to track down the TARDIS -- although Lane can't make sense of the readings he's getting.

Romana and Adric search the TARDIS stores for spare parts for K9, whose memory wafers have aged to dust. K9 spouts random snippets of nonsense interspersed with occasional attempts to warn them about the approaching humans, but Adric and Romana think these are all symptoms of his damage. In the course of their conversation Romana shocks Adric by suggesting that she might not be going back to N-Space with the Doctor after all. Before she can elaborate they see the slavers approaching on the scanner screen. Outside, Rorvik examines the police box and orders Packard to break in...

The Doctor evades the robot's first strike, but as it pursues him through the hall another one comes to life. The two robots corner him and strike out -- but hit each other with their axes, cutting themselves dead. The Doctor takes the opportunity to harvest spare parts from them for K9, but as he tries to remove one of the robot's memory wafers it suddenly begins to speak. "No Tharil shall outlive the day of the feast!"

Romana and Adric, remembering Biroc's warning, decide not to trust the new arrivals, and Romana goes out to investigate, ordering Adric to stay where he is. The slavers claim to be traders having trouble with their warp drive, but she realizes that they too must be time-travellers; they couldn't be here if their ship didn't travel between the timelines. She offers to take a look at their damaged warp drive, unaware that Rorvik has come to the conclusion that she too must be a time-sensitive like the Tharils. Adric, suspicious, decides to follow them, using K9's mass detection sensors to pinpoint the location of the greatest mass density in the void. But K9's efficiency has now deteriorated below zero optimum; in other words, he's worse than useless. Realizing that K9's circuitry calculates by triangulation, Adric orders K9 to stay put while he sets off with one of K9's sensors, but as soon as Adric is out of sight, K9 forgets the order. Once the error in his calculations is corrected K9 sets off by himself, leaving Adric lost in the void.

Romana is taken back to Rorvik's ship and is shown to the bridge -- where Rorvik informs the crew that he's found them a new navigator. Romana struggles but is forcibly strapped into the navigator's chair, which is then powered up. Despite the obvious pain it's causing her, she manages to visualise the stone Gateway, confirming Rorvik's suspicions that she's a time sensitive. Romana loses consciousness, and Rorvik and his crew set off in search of the Gateway, leaving Aldo and Royce to break one of the Tharils out of storage to navigate once they've found the way out. Since their bonuses aren't at risk, Aldo and Royce decide to try reviving the Tharil themselves, but they aren't up to the task and only manage to electrocute it. They cut the power and flee, leaving the Tharil for dead.

The Doctor learns more about the Gundan robots, which were built by slaves to navigate the time winds and kill the brutes who ruled. The Gundan runs out of power before it can explain how to escape the Gateway, but at that moment K9 arrives, and the Doctor connects his power cells to the Gundan. Rorvik and his crew arrive and hold the Doctor at gunpoint, forcing him to continue working on the Gundan. They learn that the Gateway was once the centre of an empire of slavers who descended from the Time Winds on unsuspecting worlds; but their slaves learned the secret of the Gateway and built the Gundans to strike at their vicious rulers. Before the Gundan can reveal the secret, the other Gundan comes to life, chops off its head and walks straight through one of the mirrors. Rorvik, convinced that the Doctor knows more than he's saying, try to force him to reveal the rest of the secret -- but while backing away from the slavers, the Doctor falls through a mirror and vanishes, and nobody else can penetrate the mirror to follow him; not even K9.

The Tharil, Lazlo, is just gesturing for Romana to remain calm while he frees her from the navigator's chair. He manages to do so, but Aldo and Royce approach, searching for him, and Romana sends him to hide while she pretends that she is still confined. Rorvik then contacts Aldo and Royce and orders them to fetch the M-Z out of storage. Sagan also tells them to bring lunch for the others.

The Doctor is in a stone hallway on the other side of the mirror, watching as Rorvik tries and fails to break through. Biroc, who seems to have been waiting for the Doctor to arrive, explains that the mirrors are a gateway to E-Space for time-sensitives; the Doctor was able to pass through because his hand was touched by the Time Winds. The Doctor's hand is now healing, and K9 will be repaired as well if he passes through the mirrors -- but since K9 is not organic, the repair will not be permanent, and once through the mirrors he must stay on this side. Biroc walks away as the Doctor tries to work out what's happening, and the Doctor follows him to an elegant mansion frozen between the striations of the timelines.

K9's condition continues to deteriorate, and he follows Packard and Lane back to their ship, requesting orders. They try to ignore him, instead commenting to each other on the odd readings from the mass detector and the fact that it seems a shorter distance returning to the ship than it did going out. K9 takes their discussion seriously and begins analysing the situation. Adric sees them and follows them back to the ship, where he is reunited with Romana. On Rorvik's instructions, Lane returns to the damaged part of the hull to determine whether it's safe to use the engines to back-blast through the mirrors. Romana and Adric follow him -- and discover that the hull is made of the incredibly dense dwarfstar alloy. K9 then arrives, spouting warnings of a dimensional instability and the imminent contraction of the null environment, and while Adric pursues him, trying to shut him up, Lane and Packard recapture Romana.

Romana is taken back into the ship as a prisoner, but Lazlo is waiting for her inside. He rescues her and helps her to escape from the ship by phasing into a different timeline. Adric and K9 hide aboard the MZ as the others carry it back to the Gateway -- and the slavers are puzzled that it hardly seems to take them any time at all to make the journey. Rorvik tries to motivate them to do something with the MZ, but instead they settle in for lunch first, much to his anger and frustration. When Romana and Lazlo enter the hall, walk right past him and vanish through a mirror, everybody's too startled to do anything until it's too late.

The Doctor tries to explain to Rorvik that the mirrors are only gateways for time-sensitives, but Rorvik doesn't believe him. K9 arrives spouting further warnings, and this time the Doctor listens to him and realizes that he's not just spouting gibberish. The slaver ship is made of dwarf star alloy, the only material dense enough to pin down time-sensitives, and the ship's incredible mass is causing the dimensions of the Gateway to contract. Soon this entire microcosm will reach the mathematical vanishing point and collapse in upon itself. Rorvik insists that the Doctor is just playing for time and forces him up against the mirror, threatening to shoot him if he doesn't let them through. The Doctor sees Biroc on the other side of the mirror and agrees that the Tharils have suffered enough for their crimes -- but is surprised when Biroc tells him to do nothing about it.

Before Rorvik can shoot the Doctor, Adric arrives and turns the MZ on the slavers. The Doctor backs the slavers up against the wall and tells them that the MZ's on automatic, and by the time the slavers remember that it doesn't have an automatic setting the Doctor and his companions have escaped. The slavers begin to follow them but are brought up short by the sight of their ship, looming nearby -- proof positive that the dimensions of the Gateway are indeed contracting. They return to the banqueting hall to fire the MZ at the mirrors, but the mirrors reflect the energy back to its source, destroying the MZ. Rorvik refuses to give up and decides to use the ship's engines to blast his way through.

As the Doctor and his companions ponder their next move, Rorvik and his crew manoeuvre their ship around until the engine jets are facing the archway. The Doctor realizes what Rorvik intends to do but knows that the mirrors will simply reflect the backblast energy as well, destroying everything in the Gateway. Although the explosion should theoretically kick a dematerialized TARDIS across the timelines back into N-Space, the Doctor and Romana refuse to abandon the Tharil slaves to their fate. Adric remains behind in the TARDIS while the Doctor and Romana set off for the damaged area of the slave ship, planning to use Biroc's abandoned manacles to short out the power couplings.

The ship begins to power up for the back-blast, but Rorvik can't stand the waiting, and orders his men to start breaking Tharils out of storage; eventually one will survive, and they can use it as their new navigator. The first few Tharils are electrocuted due to power fluctuations, and Rorvik heads for the damaged area to see if he can stabilise the power. Instead he finds the Doctor and Romana attempting to sabotage his ship, and tries to stop them. Biroc appears and stops the Doctor from doing anything further, and this time, the Doctor understands. Biroc phases the Doctor and Romana back out along the timelines to safety, while the gloating Rorvik corrects their sabotage; he no longer cares what happens as long as he actually gets something accomplished. Meanwhile, Lazlo finds Sagan processing the Tharils, electrocutes him with the revival equipment, and begins to awaken the remaining Tharils.

The Doctor and Romana return to the TARDIS with Biroc, but once there, Romana tells the Doctor that she's decided to remain and help Biroc to free his people. The Doctor gives her K9, who will only function on the other side of the mirrors, and returns to the TARDIS to await the back-blast. Biroc takes Romana and K9 through the mirrors to safety, and the TARDIS dematerializes just as the slaver ship fires up its engines. Just as the Doctor had predicted, the energy is reflected back and the archway and the slaver ship are torn apart by a series of shattering explosions. But the Tharils aboard the slaver ship survive by phasing into a different timeline, and once the explosions have died down they walk through the mirrors back to their own world.