The Bells of Saint John is the seventh episode of the seventh series of Doctor Who, starring Matt Smith as The Eleventh Doctor and Jenna-Louise Coleman as Clara Oswin Oswald. The seventh series was split into two parts, with this episode being the opening of the spring premiere. It continues the plot found in the Christmas Special The Snowman, with Jenna Louise-Coleman playing the same character, only mysteriously enough alive one hundred years later. The entire mystery of Clara Oswin Oswald and her identity being the main thrust of Series Seven, and the major point of this episode, other than the Doctor's usual weekly mission to fight aliens.
The Doctor is hiding in a monastery in the middle ages, painting pictures of the mysterious Clara, who he saw "die" in Victorian London. The eponymous Bells of Saint Johns ring for him: the telephone on the TARDIS, which we last saw ringing in The Empty Child, calls, and it is Clara calling him from contemporary London, because she is having computer problems and "a lady in a shop" gave him The Doctor's phone number. So The Doctor returns to modern London, only to find that Clara's computer problems are (of course) just the tip of an iceberg where an alien invader (of course) is attempting to control people by sucking them into Wi-Fi networks in a Matrixy fashion. The plot isn't particularly suspenseful or mind-bending, but it does involve some fast action and surprise twists, as well as a motorbike ride up the side of a skyscraper. And of course, some character development between The Doctor and his new companion.
Starting in Series 4, and climaxing through Series 6, and still ongoing, much of Doctor Who's plotting has been about the relationship between The Doctor and River Song, a mysterious figure who jumps around his timeline and whose abilities and origin were major plot points. This was seen by at least some fans as overdone. And yet, with this still in progress, the series introduces a second character who will have a (somewhat) romantic relationship with The Doctor, and who also jumps around in his timeline and is a mystery. I am not sure that this will turn out for the best. However, on the other hand, the series does know how to do its emotional fanservice: because despite being a fairly typical contemporary woman, there is something about Clara that causes the universes' most powerful being to work himself up into a mopey frenzy of consternation and concern over her. A message that perhaps seems attractive to at least half of the show's fanbase.