Sherlock and Watson investigate a murder which appears to have caused a plane crash. Meanwhile, Sherlock's father comes to town.
Tropes:
- Death Glare: From Sherlock to Joan when she asks him about the significance of "Irene".
- Death in the Clouds: Sherlock deduces that Hank Gerrard had been murdered with a wrench. He initially believes that Hank had been murdered while the plane was in flight, but soon discovers that the plane had been sabotaged and deduces that Hank had been killed when he walked in on the saboteur in the hangar, who then hid the body on the plane so it would be lost with the others. The killer actually wanted to kill the pilot and have the plane lost at sea, but the extra weight of Hank's body caused it to crash while still over land.
- Disability Alibi: After determining one of the victims was dead before even boarding the plane, they look into a man who was seen in a photo arguing with the victim outside of the hanger, who Watson notes has an insulin pump. When they speak to this man, they determine by the way he fumbles around with his pill bottle that he is not capable of beating a man to death.
- Tattoo Conceals Scars: While posing as Sherlock's father, Alistair mentions that one of Sherlock's tattoos covers up a scar from a childhood incident where he broke his wrist badly enough for the bone to break the skin. Shortly afterwards, Joan notices the scar on his wirst and Sherlock says it's from a childhood accident. This tips Joan off to the fact that while Alistair isn't Sherlock's father, he's also not some actor hired to play the role (as Sherlock claimed) because an actor wouldn't know such a personal detail.
- Vehicular Sabotage: The killer causes a fatal plane crash by putting sand in the plane's fuel tank.
