Why can't we go anywhere pleasant? Where there's no fighting, just peace and happiness?The TARDIS lands on the surface of the sea and the travellers use a rubber dinghy to get ashore, where they are promptly shot by tranquiliser darts and taken prisoner.They have arrived in a restricted area, a natural gas refinery, and there have been a series of problems in the pipes. One of the engineers insists that a rhythmic beating sound heard in the pipes must be a living creature.It turns out that a creature of living weed has made its way into the pipes and can secrete either a gas or a foam that enables it to control the minds of anyone who comes into contact with it. It seems intent on establishing a colony based on the refinery.The Doctor discovers that the weed is repelled by high-pitched noises when Victoria's screaming drives it away and he uses a recording of the screaming to defeat the weed creature.Victoria, having had enough of the dangers of travelling with the Doctor, decides to stay with the family of one of the workers, Harris. Jamie is sad to see her go, but the Doctor understands.This story also sees the debut of the Sonic Screwdriver, the Doctor's signature gadget, which is used as a one-shot item to undo a release valve. It is also the only Second Doctor serial to not begin with "The".
— Victoria never did get it
- Adaptation Expansion: The novelisation includes an extra scene near the end for Jamie and Victoria. Jamie goes to Victoria's room where he confesses his true feelings for her. Victoria admits her love for Jamie but she explains her tiredness for danger and death and wishes to remain with the Harris family.
- All There in the Manual: First names not present in the televised versions are included in the novelisation: Frank Harris, Mick Carney, Pieter van Lutyens, David Price and Ronald Perkins.
- Robson's wife, Angie, is said to have died in a car crash when he was driving twenty-two years previous.
- The Doctor explains that the Weed Creature transported Maggie to the control rig by encasing her in its foam.
- Attack of the Killer Whatever: Killer seaweed. It's much scarier than it sounds.
- Brown Note: Victoria's screaming defeats the weed creature.
- Everybody Lives: The Weed does not kill, it posesses. When it is destroyed, its victims recover.
- Everyone Calls Him "Barkeep": The Chief Engineer is one of the largest guest roles in the story. But he doesn't seem to have any name other than "Chief".
- Exactly What It Says on the Tin: The sonic screwdriver makes its first ever appearance in the series, and the Doctor uses it... to loosen a screw. For one of the few times ever in the series, the sonic screwdriver is actually used as a screwdriver rather than an all-purpose magic wand.
- Freak Out: Robson
- Hairpin Lockpick: Used successfully by Victoria.
- I Choose to Stay: Victoria stays with the Harris family.
- It's a Small World After All: Lampshaded by Jamie.
- Lampshade Hanging: Victoria mentions that the TARDIS always seems to land on Earth.
- Mean Boss: Robson. He refuses to take advice, knows it all and rails against his subordinates for the slightest perceived failure.
- Nothing Is Scarier: This story manages to make gas, foam and seaweed terrifying.
- An advantage of the episodes not surviving is that it works very well on audio.
- Not Himself: More and more people in the complex.
- Plant Aliens: Maybe. It's never made clear whether the weed creatures are alien or not. They've been on Earth for centuries, wherever they originate.
- Puppeteer Parasite
- Reasonable Authority Figure: Megan Jones, who gives the Doctor all the help she can once she is convinced of the truth of what he's saying.
- Screaming Woman: The story puts this trope to constructive use.
- Those Two Bad Guys: Mister Oak and Mister Quill. Don't let them breathe on you.