Contains spoilers for this episode and earlier episodes.
Sherlock rejects Mycroft's request to investigate a dead government employee and stolen missile plans, then receives a challenge he cannot ignore. A phone recovered after an explosion directs him to the empty flat beneath Baker Street. Inside are a schoolboy's trainers. A frightened woman relays the bomber's instructions: Sherlock must solve the mystery within twelve hours or the explosives strapped to her will detonate.
The shoes lead Sherlock back to Carl Powers, a swimmer whose death he questioned as a child. Solving that murder releases the first hostage, but more puzzles follow. A staged disappearance and a television presenter's poisoning bring further deadlines. One captive dies after trying to describe the bomber's voice, despite Sherlock identifying the killer in her assigned case. A forged painting threatens another victim until Sherlock spots a star that could not have appeared in the original artist's sky.
John pursues the missing missile plans while Sherlock follows the bomber's trail. After recovering the stolen material, Sherlock offers it at a swimming pool meeting. John arrives wearing explosives, and Molly's apparently harmless boyfriend Jim reveals himself as Moriarty. John tries to sacrifice himself to give Sherlock an escape, but the snipers keep both men trapped. Sherlock removes the vest when Moriarty leaves, only for him to return. With guns trained on them, Sherlock aims at the discarded bomb.