Warriors’ Gate is the fifth serial of the 18th season of the British science fiction television series Doctor Who. It was written by Stephen Gallagher and was first broadcast in four weekly parts on BBC1 from 3 to 24 January 1981.
The serial is set at an intersection between the universe of E-Space and the home universe of the alien time traveller the Fourth Doctor (Tom Baker). In the serial, the Doctor and his travelling companionRomana (Lalla Ward) seek to free the time-sensitive Tharils from a group of slavers led by Captain Rorvik (Clifford Rose).
Inside the TARDIS, the Fourth Doctor, Romana, Adric, and K9, while travelling between E-Space and the normal universe (N-Space), become trapped in a white null space between the universes. Elsewhere in the void, a slave vessel, run by Captain Rorvik, has also become trapped. It uses members of the leonine Tharil race as their navigators. On becoming stuck, the current navigator, Biroc, escapes the ship and makes his way to the TARDIS on the winds of time. Biroc warns the TARDIS crew of Rorvik’s treachery before disappearing. K9’s memory wafers are shredded by the winds of time, leaving him functional but lacking long-term memories. The Doctor leaves on his own to explore the null space. He encounters some robots, called Gundans.
Meanwhile, Rorvik and his crew have discovered the TARDIS. Romana leaves to talk to them. Rorvik, believing Romana to be time-sensitive like the Tharils, dupes her into returning to their ship to examine the engines. When Romana does not return, Adric and K9 leave to recover her, but get separated; Adric eventually makes it to the ship and hides aboard, while K9 reunites with the Doctor and aids in repairing the Gundan, after which he learns from it that they were built by slaves and used to overthrow their masters in a violent battle. The Doctor’s work is disrupted when Rorvik and several of his men arrive. During a stand-off between the crew and the Doctor, another Gundan activates and walks through the seemingly-solid mirror. Rorvik demands an explanation from the Doctor, revealing he has Romana captive, but the Doctor’s only response is to walk through the mirror himself.
Aboard the slaver ship, Romana is freed by another Tharil named Lazlo, and she hides in the hull. She encounters Adric; the two work out that the ship is made from an incredibly dense dwarf star alloy that can contain the Tharils. K9 arrives, and informs the two of dimensional instability in the null space, which they attribute to the alloy, causing the space to collapse in on itself. Romana rejoins Lazlo, and takes her to the gateway and through the mirror, while Adric remains aboard the ship.
On the other side of the mirror, the Doctor and Romana are reunited with Biroc in a stable, time-locked universe. A repentant Biroc explains they were the slave masters, travelling on the winds of time in order to ravage other planets and subjugate their populations as slaves until the Gundan revolt. The Doctor and Romana are returned to the null space, and are immediately captured by Rorvik. Rorvik’s crew realise that the null space is shrinking as the distances between the gateway, the TARDIS, and slaver ship continue to decrease. Rorvik has ordered the crew to try to blast through the mirrors in gateway, believing it to be the way out, but the mirrors resist all attacks. With the gateway and ship in visible distance of each other, Rorvik resorts to one last attempt to break the mirrors by using the exhaust of the ship’s engines against them. While the Doctor warns that this action will be as doomed as the previous ones, Romana regroups with Lazlo and Adric, and together they free the remaining Tharils on the slaver ship. The TARDIS crew flee to the TARDIS as Rorvik initiates his plan—the blast from the engines is reflected by the mirrors back onto the ship, destroying it and its crew.
As the saved Tharils pass through the mirror, Romana announces that she will be staying with them, having become empathetic to their plight and not wanting to return to Gallifrey. The Doctor gives her K9, as passing through the mirror will restore his memory but he will be unable to return. The Tharils, in exchange, provide the Doctor with information on how to leave the void back to N-Space.