The Enemy Of The World really is a wonderful example of Doctor Who in the late sixties, and I heartily recommend it to everyone! Go and watch it if you haven't already, and then come back and read the rest of this post, because it's rather spoilerrific (as the trendy young nerds on the internet say).
You see, great though it is to have the serial available on iTunes (so great, indeed, that I betrayed my modern-technology-hating principles and downloaded iTunes), the rush to release it seems to have led to the episode descriptions being written by someone who knows the plot (maybe they read the novelisation back in the days when that was the only way to experience old Doctor Who stories) but hasn't actually watched the episodes themselves. These are the summaries of the episodes that you'll see if you buy the iTunes version:
The TARDIS lands on a barren beach, where it is immediately fired upon by a hovercraft. Rescued by helicopter pilot Astrid, the Doctor (Patrick Troughton) discovers that he has arrived on Earth the year 2017 A.D. and that he is the exact double of a would-be dictator called Salamander.
The Doctor has arrived on earth in the year 2017 A.D. Rescued from certain death by helicopter pilot Astrid, the Doctor is informed by Astrid's boss, Kent, that the world is on the verge of domination by a charismatic scientist/politician named Salamander. The fact that the Doctor is a dead ringer for Salamander leads to several even more perplexing plot elements: who among Kerr's staff can be trusted not to betray their comrades to Salamander's minions, and is Salamander truly the despotic villain that he is made out to be?
The Doctor impersonates his lookalike, would-be dictator Salamander. He does this to penetrate a research station controlled by Salamander's minions, the better to find out if the villain truly is a villain - and if so, who his most fervent (and dangerous) followers may be. This has a trickle-down effect on the Doctor's travelling companions, Jamie and Victoria.
Giles Kent is revealed to be a traitor, in league with would-be 21st century dictator Salamander. Meanwhile, the Doctor - who happens to bear a striking resemblance to Salamander - has infiltrated the dictator's research station. Alas, the station is blown up, with the Doctor apparently still inside. Conspicuous by their absence throughout the proceedings are the Doctor's companions, Jamie and Victoria.
Traitorous scientist Kent has blown up the research station infiltrated by the Doctor. Not long afterwards, 21st century dictator Salamander, who bears a startling resemblance to the Doctor, makes an appearance. But is Salamander really the Doctor - or is it the other way around?
Having been thwarted in his plans to rule the world, Salamander tries to make a quick getaway by posing as the Doctor. Manning the controls of the TARDIS, Salamander tries to escape - but will he be successful?
The first episode summary is more or less accurate - the TARDIS isn't fired upon by a hovercraft, the Doctor and his companions leave the TARDIS to play on the beach, they're spotted by three men in a hovercraft who themselves fire on the Doctor and co and chase after them on foot. The year isn't specified either, but Astrid's helicopter licence expires at the end of 2018, and the story was written in 1967, so it was probably intended to be set fifty years in the future.
The perplexing plot element "who among Kerr's staff can be trusted not to betray their comrades to Salamander's minions" certainly is perplexing - even if you read 'Kent' for 'Kerr' (the actor who played him was Bill Kerr), this question doesn't really come up in the course of the episode, or anywhere else in the story.
Then it really gets a bit strange with the descriptions of episodes 3 to 6. They omit almost everything that happens in those episodes, and summarise the final bit of the storyline as if it's spread across the final four. In the actual show, the Doctor prepares to infiltrate the research station at the end of episode 4 and doesn't actually get in there until episode 5. The revelation about Kent (which isn't that he's in league with Salamander - he wants to kill him, as he's been saying all through the serial - but that the two of them worked together on Salamander's evil plan, years ago) happens at the start of episode 6, then it's revealed that the person Kent thinks is Salamander at that moment is in fact the Doctor, THEN, near the end of the final episode, the station is blown up, and in the brief final scene Salamander tries to get away in the TARDIS.
So on the plus side, if you're watching it on iTunes after reading the episode summaries, major parts of the plot will be a complete surprise to you. But on the other hand, you'll be expecting some things to happen an hour or more before they actually do...