While trying to return Ian and Barbara to the twentieth century, the TARDIS doors apparently malfunction and begin opening in the middle of materialization. The Doctor is quiet upset by this mishap, but can't seem to find anything damaged by the faulty maneuver. The TARDIS scanner blows a tube and the groups only way to find out where they have landed, is to just go outside.

The foursome splits up into pairs and begin exploring. Each group finds what appear to be giant insects. At first they come to the conclusion that they have landed on a planet where everything is giant size. With further investigation. however, they realize that they are actually back on earth but they have been shrunk down in size. The TARDIS has in fact landed between the stones of a backyard patio.

As it happens, a murder is about to happen in this very same backyard. Two men are having a disagreement over a pesticide called DN6. One of the men believes the chemical is too dangerous, it not only kills undesirable insects, but the beneficial ones too. He is about to recommend shutting the production of the chemical down. Unfortunately, there is too much money to be made from the new chemical, and the whistle blower ends up dead.

The TARDIS crew begin finding evidence of the DN6 that has been used in a trial run in the backyard. Dead insects can be found everywhere. It isn't long before they also come across the dead body of the murder victim. Before they can investigate further, Ian and Barbara are forced the separate from the Doctor and Susan by a giant pair of feet. The two teachers take refuge inside a giant briefcase. Unfortunately, it is the very same briefcase the murderer has just picked up.

Ian and Barbara are taken inside the house and deposited in some sort of laboratory. They crawl from the briefcase and investigate their surroundings. Barbara inadvertently picks up a giant size piece of wheat before realizing that it has been sprayed with DN6. Before Barbara can tell Ian of her mistake, they hear Susan's voice calling to them. The Doctor and Susan have crawled up a drain pipe into the laboratory sink to find their missing friends.

Narrowly avoiding being washed back down the drain, the crew reunite and find a giant notepad with the chemical composition of DN6 written on it. From this information, the Doctor realizes that the chemical never breaks down, and if used would eventually get into the water system and even become harmful to humans. They must do something to stop the chemical from being used.

The Doctor spots a giant size phone and believes they should try to use it to warn the authorities. After great struggle, they are able to life the receiver but they soon discover it was a futile effort. At their size, their voices can not be heard by the operator on the other end. It does, however raise the suspicions of the local constable, and he decides to go around to investigate.

In an attempt to cause trouble, the Doctor and his friends manage to turn on the gas spigot in the lab and light it with a giant match. They have also moved a can of the insecticide in front of the open flame hoping the heat will cause the can to explode. Fortuitously, the can does explode, taking the murderer by surprise and temporarily blinding him just as the constable shows up to arrest him.

As Barbara collapses from the poison on her hands, the crew realize they must return to the TARDIS in a hurry. Her only hope is to be returned to normal size where the poison would be diluted. The Doctor attempts to reverse the process that had left them in a miniature state. Thankfully, the trick works and the TARDIS returns them all to normal. After a good scrubbing, they are all ready to continue their travels.