The episode begins with a dire warning from a young man, pleading with people not to click on the odd alien symbols in their computer's WiFi connection. Doing so will result in a person being spirited away to some unknown place from which they cannot escape. He knows this because he, and hundreds of others, suffered the same fate.
Meanwhile, unable to solve the mystery that is Clara Oswin Oswald, the Doctor holes himself up in a monastery in 13th century Cumbria. Then one day, he gets a call from her on the TARDIS phone. Like, the real phone that's not supposed to ring. She says she can't find the WiFi on her laptop, and got his number from someone who said he could help. The Doctor tries his best to help her, but she is pretty much all thumbs when it comes to computers.
When he tracks her down to modern-day England – that is, our modern day – he finds her unconscious in her foyer. She is in the process of being downloaded by a "walking base station," which looks mostly like a person, but has something like a metal bowl in the back of his or her head. The Doctor counters the download and gets her placed back into her own body, which is now suddenly much more tech-savvy than it was before. She doesn't seem to recognize the Doctor, but she helps him hunt for the aliens who are behind the attack.
The responsible party is the Great Intelligence, last seen in the Christmas Special making snowmen. The Great Intelligence has taken over the world's WiFi networks in order to control and find the human minds it needs. To that end, it has employed a Miss Kyzlet, who runs an office in The Shard filled with programmed human beings. They're eager to get Clara back, and manage to find her just as Clara finds them. This time, they complete the download by using a base station that looks like the Doctor.
The Doctor demands Kyzlet free Clara, but she insists that can't be done without freeing everyone, which her boss just wouldn't like. So the Doctor commandeers his lookalike base station, storms The Shard and downloads Kyzlet. Once she's inside the computer, she orders her employees to free everyone. The Great Intelligence flees, and Kyzlet and her employees revert to their normal, non-programmed selves.
At the end, the Doctor invites Clara to take time off from her job as a nanny and travel with him. She plays a bit hard to get, though, and doesn't accept the offer right away, telling him to come back and ask her again the next day. Hopefully, he's learned to be a bit more punctual after all his missed connections with Amy. Stay tuned for the full weecap.
"This is a warning to the whole world," says a young man. He appears to be sitting in an office or perhaps just a very well-organized living room. "You're looking for WiFi," he goes on. "Sometimes you see something a bit like this." He holds up a piece of paper with some symbols that look like a cross between Tetris shapes and cuneiform. All over the world, people casually log into WiFi networks at home, on the train, at restaurants. "Don't click it," he says. "Once you've clicked it, they're in your computer." Either people don't see his warning, or don't pay any attention to it. One computer user after another clicks on the string of symbols while searching for a wireless network. "They can see you, and if they can see you, they might choose you. And if they do, you die." The people we've been watching up to this point all drop dead, their computers near at hand. "In 24 hours, you're dead." He says people's souls are being uploaded to the internet. "Sometimes you can hear their screams on the radio, on the telly, on the net..." The dead people pop up in online videos saying "I don't know where I am" in various languages. The young man who's been warning us is one of those people who is now trapped in the internet. Other victims fill an entire wall of video screens, each calling for help. Cue opening credits.
Far from the computer age, we drop into a monastery in 13th century Cumbria. A young monk pounds on the outer door. "Wake the abbot! The bells of Saint John are ringing!" The old abbot leads him and several other monks deep into the stony cloister. A man in a hooded robe sits surrounded by candles and canvases. "The bells of Saint John are ringing," says the abbot. The hooded figure stands up, lowers his hood and reveals himself to be... the Doctor. Of course. Why do they even bother with these "suspenseful" reveals? "I'm going to need a horse," he says. While he scampers off in search of a horse, the monks muse over a canvas bearing Clara's portrait. The Doctor has been holed up here, trying to figure out how to find her.
He's about 800 years off the mark, because she's currently somewhere in 21st century England, trying to get hold of tech support on the phone while she hustles a brood of children to get ready for school. (They're not hers, as they're quick to point out.) As the kids and their father head out the door, Clara stops a boy to check on the book he's reading. It's by Amelia Williams nee Pond, just in case you were wondering how Amy was getting along in the past. Her job done for the morning, Clara flounces upstairs and turns on her laptop. Her wireless network connections offer her those Tetris symbols. She ponders it while she waits for tech support to answer the phone.