The Doctor turns up towards the end of a first contact situation. A treaty is about to be signed. "Historical records" have been exchanged, showing how good, wise and benevolent the Daleks are. At first the Doctor doesn't get a clear view of the records, or the ship making contact, but then he hears the classic Dalek sound...
The Daleks are, for them, weak. It's early in their re-expansion after their near destruction at the hands of the Doctor. They need Dalekanium to continue expanding and that's why they don't go in gun blazing. This planet is naturally rich in a precursor element necessary to make Dalekanium and technologically advanced enough to defend itself effectively.
We see the Daleks gain the trust of the planetary leadership, but a significant minority oppose the treaty, despite the apparent benefits. Rather like in "V", the Daleks offer advanced technology, a way for the elite to retain control and to help "deal with" the opposition.
After much action and plot detail, in which we see Dalek atrocities, death camps, brainwashing, strip mining, etc, etc, the Daleks' influence grows till they betray the leadership and take control of the planet themselves. The Doctor and other opponents are eventually imprisoned. About to be executed in one of thousands of death camps, they are rescued by other opponents. The Doctor rallies the planet, the Daleks will be overthrown and expelled from the planet.
The Daleks have gone. Celebrations begin, but the Doctor isn't so sure all is won. Planetary defences are down, sabotaged by the Daleks or collaborators.
The Daleks destroy the planet from orbit. It's a moral victory for the Doctor. He's slowed their expansion by denying them Dalekanium but couldn't save the planet. It's a Pyrrhic victory for the Daleks, they collected some Dalekanium, but not enough. They beat the opposition, but destroyed the planet in doing so. And the Doctor escaped. again.