The Doctor and Gerald are at a planet known for its caves and gems, as well as the unique species, the Gemyno, beings made entirely out of gems. The duo are having a conversation with one of the Gemyno when a ship lands directly on top of a crowd of Gemyno, crushing them into shards.
Cue title sequence: Beginning
As it turns out, the ship is full of space pirates (may or may not be related to those people from Journey to the Center of the TARDIS) looking to steal the Reamaer, the gem that provides life for the Gemyno, and soon enough, this adventure turns into a hostage story.
Gerald tries to negotiate with the pirate' leader, only to be shot in the gut by said leader. At this point, the Doctor has had enough of the sound of gunfire and goes absolutely insane. Here, we finally see the trauma caused by his regeneration. He nearly bashes the shooter's head in before the shooter's son tackles the Doctor.
Seeing this as their opportunity, the Gemyno are able to escape as the Doctor engages in a 1v6 fight that ends when another gunshot is heard, bringing attention back to the leader, who was just killed by a standing Gerald. The pirates point their guns to Gerald before they are stunned by the returning Gemyno. The Doctor and Gerald return to just exploring the planet before getting bored and returning to the TARDIS. Gerald leaves his coat on a chair, and as the Doctor picks it up, the Reamaer falls out of one of the pockets. The Doctor looks to Gerald's direction in confusion.
Cue Credits: The Doctor Will Return in: Meddle