Susan and Barbara are taken to safety by a guerrilla movement while Ian and the Doctor eventually discover signs of the Robomen and eventually the Daleks. This is the first appearance of the Robomen, and in this incarnation they are humans enslaved by Daleks, made to wear Robomen helmets, and used by the Daleks as foot soldiers to assist in the enslavement of humankind. The Doctor and Ian are taken by the Robomen to the Dalek fortress at which is located a flying saucer they'd previously seen flying over London.
How did Daleks come to Earth in 2164 when their origin lies a million years into the future on a planet far far away? That's not really explained, and no we can't blame this on George Lucas.
One purpose for the Saucer is housing devices that convert humans into Robomen. And it seems that Ian and the Doctor are chosen to become Robomen.
They learn that the Daleks are using enslaved humans to run a mine in Bedfordshire but it's not known what they're digging for. They're just forcing humans to dig, and dig, and dig. Eventually everybody makes it to the mines and we see the worst of slave labor conditions. Along the way Barbara and Ian become separated into their own separated journeys. Barbara takes up with another woman, they steal a fire truck from a museum and drive it out of town until the Daleks see them and destroy the truck. Later they find an isolated hovel where a pair of women are living, the women earn a living making clothes for the prisoners in the camp, and they earn a little bit of extra food by betraying Barbara to the Robomen. Ian on the other hand is stuck on the saucer, hiding with a compatriot, and they eventually get to the mines.
The Doctor and Susan are with the surviving rebels and making their way towards the mines. Along the way Susan falls in love with one of the rebels and there is a little bit of a competition in her affection for the Doctor and the affection for her new-found love.
Eventually they get to the mines and begin confronting the Daleks there. Barbara had been taken prisoner and made to work deep in the mines. Ian is a free agent who works his way into the mines as well. They discover that the Daleks are simply mining a hole deep, soley for the purpose of mining a hole, and that there's something about dropping a missile into the mantle of the earth. It's eventually explained that the Daleks wish to remove the Core of the Earth, replacing it with a 'power system', which will allow them to pilot Earth anywhere in the Universe.
However Ian is able to obstruct the placing of the bombs which would enable that theft. Instead the bombs explode well above the Mantle with a very different effect. Namely, a volcano eruption. In England.
The eruption happens to catch the escaping Dalek spacecraft, killing the Daleks. End of problem everybody's saved.
They all return to London and end up at the TARDIS. There's a general air of victory with everybody discussing things they'll do to remake human society. But there is a very odd thing which happens within the companionship of the TARDIS.
Susan has clearly fallen for this fellow in the resistance. He has asked her to stay and marry him. Susan wants to, but doesn't want to. The Doctor then does a very odd thing. He goes into the TARDIS, locks the door, and speaks to Susan over a loud speaker giving this speech: "During all the years I've been taking care of you, you in return have been taking care of me. You are still my grandchild and always will be. But now, you're a woman too. I want you to belong somewhere, to have roots of your own. With David you will be able to find those roots and live normally like any woman should do. Believe me my dear, your future lies with David and not with a silly old buffer like me. One day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and prove to me that I am not mistaken in mine. Goodbye Susan. Goodbye my dear." According to Carole Ann Ford, she got tired of the role of Susan because the producers of the show would not let her expand and develop the character.
Uh.. either this is a "if you love them set them free" moment or else he's abandoning Susan. I'm not sure which. It's clear that Susan is a Time Lord so why would he leave a Time Lord to have a human life? This doesn't make much sense.
The serial was the basis for the Peter Cushing Doctor Who movie, Daleks - Invasion Earth 2150 AD.
In The Stolen Earth the Tenth Doctor makes a vague reference to someone trying to steal the Earth before. Hmm.. this series has to be what he referred to.