After working all night in the UNIT lab on a device to help locate the Master, the Doctor has a dream in which he sees a trident-shaped crystal and his arch enemy (Roger Delgado). When he wakes, Jo mentions Atlantis and the Doctor's interest is piqued. He summons the Brigadier (Nicholas Courtney), who not interested as he is due to attend a demonstration of a device called TOMTIT - Transmission Of Matter Through Interstitial Time - at the Newton Institute in Wootton just outside Cambridge. 

The man behind TOMTIT is the Master, posing as Professor Thascales. He hypnotises Doctor Charles Percival (John Wyse) at the Institute when he becomes suspicious. At the heart of TOMTIT is a trident-shaped crystal. With the Master gone, his assistants, Doctor Ruth Ingram (Wanda Moore) and Stuart Hyde (Ian Collier), decide to run a test before the demonstration. 

The Doctor has built a time sensor which detects distrubances in the time field and when Ruth and Stu start their test, it starts to register. The Doctor is able to identify the rough location of the disturbance and go out in Bessie in case it starts again. 

The Master, seeing that UNIT personnel will be at the demonstration, disguises himself in a radiation suit. The Doctor picks up the signal and rushes towards the Institute. At the climax of the demonstration, the Master shouts out a summons to Kronos and then hurries out. The Doctor and Jo arrive, but Jo is caught in a time field and frozen. The Doctor gets Ruth to reverse the temporal polarity and the machine stabilises. Stu, however, has aged into an old man. The Doctor realises that Thascales is the Master and Stu raves about the crystal and seeing Kronos. The Doctor goes with Ruth to check the crystal and recognises it as the Crystal of Kronos - the Master is trying to capture Kronos the Chronovore. The Doctor powers the machine up and Benton (John Levene) is unable to move the crystal. This is because it isn't really there at all. It is linked to or is the same as the crystal in Atlantis. 

In Atlantis a neophyte (Keith Dalton) sees the crystal glowing and reports this fact to Krasis (Donald Eccles), the High Priest. 

The Master knocks Benton out and activates TOMTIT. Krasis is transported from Atlantis to the Institute. Benton manages to escape and warns the Doctor. The Master uses a medallion from Krasis to enter the equations to summon Kronos (Marc Boyle). Kronos appears as a giant white bird-like creature and Stu returns to normal as time feeds back. The Master cannot control the Chronovore and returns Kronos to the crystal after it had devoured Percival. Krasis tells the Master that this crystal is only a fragment of the true crystal which is still in Atlantis. 

In Atlantis, Hippias (Aidan Murphy) informs King Dalios (George Cormack) that he saw Krasis and the crystal vanish. Dalios, who claims to be over 537 years old, shows Hippias the true crystal which is guarded by the Minotaur (Dave Prowse). 

The Master drains the energy from the crystal so he can take it to Atlantis. UNIT troops headed by Captain Yates (Richard Franklin), are on their way, called for by the Brigadier, and the Master summons a knight in armour (Geoffrey Powell), Roundheads (Dave Carter [Roundhead Officer]) and a World War II flying bomb to try and stop them. The Doctor travels out to meet them as they are also bringing the TARDIS. To try to stop the Master from leaving, the Doctor moves his TARDIS inside the Master's, however the Master's ship also ends up inside the Doctor's. The Master activates TOMTIT and enters his ship. The Doctor is unable to make the Master listen to him over the TARDIS' communication system and so leaves his TARDIS to confront the Master directly. The Master summons Kronos to devour the Doctor and then separates the TARDISes in the vortex, sending Jo in the Doctor's ship spinning away. 

The Doctor contacts Jo through the TARDIS' telepathic circuits and she is able to bring him back unharmed. Back at the lab, the Brigadier and his UNIT troops are frozen in time and Stu and Ruth try to reverse the effects of the machine but manage instead to transform Benton into a baby (Darren Plant). 

The Master arrives in Atlantis and starts making plans to obtain the crystal. King Dalios won't respond to him and so he seduces Queen Galleia (Ingrid Pitt) instead. When the Doctor and Jo arrive, Dalios is sympathetic. Jo follows Hippias and Krasis and is thrown into the labyrinth where she confronts the Minoraur. Galleia's handmaiden, Lakis (Susan Penhaligan), alerts the Doctor and Dalios and the Doctor manages to defeat the Minotaur but not before it has killed Hippias. Returning to the palace, the Doctor and Jo find that the Master is now King. He has the Doctor and Jo thrown in a dungeon. They are shortly joined by Dalios who, before he dies, tells the Doctor he must save the world. 

Queen Galleia introduces the Master as King but is unaware that Dalios is dead until the Doctor reveals this. Krasis operates the TOMTIT device and summons Kronos who starts to destroy the city. The Master takes the crystal and heads for his TARDIS but Jo leaps on him and is carried along as he leaves. The Doctor chases in his TARDIS. He intends to ram the Master's ship but cannot bring himself to do it as this action will kill Jo. Jo makes the decision for him and operates the appropriate controls in the Master's ship. 

The Doctor and Jo find themselves in a featureless void where they come face to face with Kronos who now looks and sounds like a woman (Ingrid Bower). She is grateful to the Doctor as he has released her and offers to grant his desires. All the Doctor and Jo want is to go home. She intends to punish the Master, however. When he pleads for his life, the Doctor intercedes and asks that he go free - the Doctor will return him to the authorities on Earth. Kronos agrees. The Master siezes the opportunity and escapes in his own ship. 

At the Institute Ruth finally manages to reverse the time field and the Brigadier is freed. The machine explodes and Benton is returned to his normal age.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}