Story 81 - Planet of Evil

In which the Doctor and Sarah are returning to UNIT HQ when the TARDIS picks up a faint distress signal. This takes them to the densely-jungled planet of Zeta Minor, on the outermost fringe of the known universe.
A scientific expedition from the planet Morestra is here. Some powerful and invisible force has attacked the team members - draining their bodies of all energy. Only one man survives - the leader, Professor Sorenson.
The expedition's distress signal has also been intercepted by a military spaceship from their homeworld, which was on its way to check on them. In command is Salamar. Second in command is the more experienced Vishinsky.
The Morestran ship lands and the crew find the TARDIS. It is transmatted onto their craft with Sarah on board, and she is captured when she emerges.
The Doctor is caught soon after in the expedition's quarters. He and Sarah are accused of being responsible for the deaths. Things are not helped when one of the Morestran soldiers is killed shortly after they escape.

Sorenson had come to this primitive world in search of a new energy source. Morestra's sun is dying. Energy crystals have been found here, located near a mysterious black pool which does not reflect light. The Doctor examines a sample of the crystals and discovers that they are rich in anti-matter energy. This planet sits on the edge of the universe of matter, and the pool forms a gateway to the neighbouring universe of anti-matter. It is home to a creature capable of straddling the two universes. It is this which has been killing the Morestrans. The crystals are only stable on this world, and to take them to Morestra would be disastrous. The whole universe could be at risk. The crystals have also infected Sorenson. He suffers blackouts, during which he transforms into a savage being. In this form, he is also capable of killing in the same way as the creature from the pool.

To stop the attacks by the creature, the Doctor goes to the pool to communicate with it. He assures the creature that none of the crystals will be removed from the planet. When the Morestran ship takes off, however, the anti-matter energy within Sorenson is enough to cause the ship to be dragged back to the surface. The professor had been using a chemical to try to stabilise his condition, but this no longer works and he goes on the rampage through the ship. Salamar shoots him with a radiation weapon but instead of killing him, multiple energy copies are created - all just as deadly. Salamar is killed. Vishinsky and Sarah are trapped in the flight deck as the ship heads for destruction. The Doctor lures Sorenson into the TARDIS and takes him to the black pool. Sorenson falls in. He is released unharmed, the contagion gone, and with little memory of recent events. The Doctor steers him towards looking at kinetic energy to solve the Morestran sun problem.

This four part adventure was written by Louis Marks, and broadcast between 27th September and 18th October, 1975.
This was Marks' third contribution to the programme, following 1964's Planet of Giants and 1971's Day of the Daleks. Two clear influences can be discerned in this story. One is Robert Louis Stevenson's The Strange Case of Dr Jekyll and Mr Hyde - in the transformations of Professor Sorenson and his chemical attempts to control them. The other - in terms of general design and the mostly invisible monster - is the classic 1956 film Forbidden Planet, which was in turn inspired by Shakespeare's The Tempest. The Anti-Matter Monster is very much based on that film's Monster from the Id. There are parallels between Sorenson's attitudes towards science and experimentation, and those of Dr. Morbius.
There is a great cast list - and all Doctor Who veterans - but perhaps the biggest star of the programme is Roger Murray-Leach's astonishing jungle set, filmed at Ealing. One of the best pieces of design work ever to feature in the show.

As for the human stars, we have a reunion from 1966's The Savages - with Frederick Jaeger, who had played Jano, appearing as Sorenson, and Ewan Solon, who had been Chal, as Vishinsky.
Three prominent crew members are played by actors who have been in the programme before. De Haan is played by Graham Weston. He had appeared in The War Games. Ponti is Louis Mahoney, who had played a newsreader in Frontier in Space and who will return to the series as the old Billy Shipton in Blink.
Michael Wisher plays Morelli. Salamar is played by Prentis Hancock, who had appeared twice during the Pertwee years and would make one further appearance with Tom Baker.
Episode endings are:

1. The glowing red outline of some massive creature can be seen as it bears down on the Doctor and Sarah...
2. Tracked by the oculoid device, Sarah and the Morestrans see the Doctor arrive at the cave of the black pool. As the Anti-Matter creature emerges, the Doctor tumbles into the pool...
3. Sarah and the unconscious Doctor are trapped in pallets which are about to eject them into space. Vishinsky refuses to operate the equipment, but Salamar forces him to eject them...
4. Sorenson returns to Morestra with Vishinsky, whilst the TARDIS carries the Doctor and Sarah to their rendezvous with the Brigadier at UNIT HQ...

Overall, quite a tight suspenseful four parter. With Harry no longer around, we really get to see why the Tom and Lis partnership is regarded so highly. Some very scary material for younger viewers - with the red outline monsters and Sorenson's hirsute, savage alter-ego. The dessicated skeletal corpses are quite horrific for a tea-time audience. Great cast. Great design. It doesn't hide its literary and cinematic references.
Things you might like to know:

• As well as playing Morelli, Michael Wisher can also be heard playing an unseen crew member - with a rather dreadful Indian accent. This will be his final appearance in the programme. Had Resurrection of the Daleks not been put back a season due to industrial action, he would have reprised his role of Davros.
• The Morestrans have transmat capabilities and yet land their ship on Zeta Minor. You would expect them to beam down a party instead. Unless it doesn't work on people, and Sarah was only protected by being in the TARDIS.
• We see the TARDIS control room for the first time since part one of Death to the Daleks. The new set was used first for Pyramids of Mars, as that story was filmed before this one. You'll note that the scanner is missing.