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
- Brown Note: Victoria's screaming defeats the weed creature.
- Everybody Lives
- 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.
- It's a Small World After All: Lampshaded by Jamie.
- Mean Boss: Robson. He refuses to take advice, knows it all and rails against his subordinates for the slightest perceived failure.
- 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.