The Doctor has a nightmare, more a premonition, of the Master being in control using a trident-shaped crystal, and it turns out he is right. His nemesis, using the alias Professor Thascales, has invented a time-device called TOMTIT (Transmission Of Matter Through Interstitial Time) at Wooton, just out of Cambridge. It basically dematerializes an object, sends it through "the crack between now and now" and rematerializes it at its destination. And yes, it does use a trident-shaped crystal. He has Dr. Ruth Ingram and her brother Stuart as his assistants.
The Brigadier is sent as a UNIT observer, but during a trial run, Stuart is aged to an octagenarian. The Master then calls for Kronos, who is a chronovore, dangerous creatures living in the time vortex that cab "swallow a life as quickly as a boa constrictor can swallow a rabbit. Fur and all!"
The Doctor and Jo arrive at Wooton because they detect the Master's TARDIS and comes upon an aged Stuart, who mentions the name Kronos. From then, it's the Doctor and Jo against the Master and Krasis, the high priest of Atlantis who unwittingly helps the Master in controlling the dangerous chronovore.
One interesting goof is the word "chronovore." "Chrono" is Greek, while "vore" is Latin. Surely "chronophage" should have been more appropriate, or "temporavore"? Another is the V-1 rocket that the Master brings through time against Yates' convoy. A farmer remembers a V-1 striking in the exact area in 1944. Unless there was more than one V-1 attack in that area, it's impossible for the farmer remembering it if it had been taken out of time. And Aidan Murphy (Hippias) has a shrill and irritating voice.
Other items: the Doctor and the Master's TARDIS materializing within each other is later duplicated in Logopolis. And the rounded wall designs in the Doctor's TARDIS is a neat improvement from the usual. In fact I wish it'd stayed that way. Jo is at her most loyal and bubbliest, and not only has a chance to wear her usual mini and long boots but a nice Atlantean dress (and surely a wig). And one must suspend some sort of disbelief over the time flow analogue constructed by the Doctor, created out of a wine bottle, two forks, two corks, keychains, and some piece of equipment. At least the Doctor's having fun.
George Cormack (Dalios) would later appear in Jon Pertwee's final story Planet Of The Spiders as the old mountaintop hermit referred to in Episode 6. Dave Prowse (the Guardian) is best known as the body of Darth Vader in the original Star Wars trilogy. The big guest star here only appears in the last two episodes, and that's Hammer horror queen Ingrid Pitt (Galleia), whose deep sultry voice is used to great effect.
The Time Monster has the reputation of being one of the stinkers of the Jon Pertwee era, but that is hardly the case. It is an engaging 6-parter, one of the best UNIT stories, and features one of the Master's boldest schemes.