Contains spoilers for this episode and earlier episodes.
Two years after his apparent death, Sherlock returns to London to help Mycroft stop a terrorist attack. He expects John to welcome him back, but finds him preparing to propose to Mary Morstan. Sherlock's surprise appearance in a restaurant brings anger and violence rather than a happy reunion. John has spent those years grieving, while Sherlock has been dismantling Moriarty's network without telling him he survived.
With John refusing to resume their partnership, Sherlock takes Molly on cases instead. An Underground employee brings him footage of a passenger who apparently disappears between stations. Sherlock eventually recognises that an entire carriage has vanished with the man. Meanwhile, somebody abducts John and puts him beneath a Guy Fawkes bonfire. Mary receives a coded warning and joins Sherlock in a motorcycle race to pull John out before he burns.
Sherlock identifies the missing passenger as Lord Moran and locates the diverted carriage beneath Parliament, packed with explosives. John joins him underground, where Sherlock claims he cannot stop the bomb and begs forgiveness. After John admits how much he missed him, Sherlock reveals that he found the off switch. Moran is arrested and the friends begin working together again. Competing accounts of Sherlock's survival leave room for doubt, while John's kidnapping remains unresolved. An unknown man watches footage of the bonfire rescue.