Borderlands: The Pre-Sequel puts you in the shoes of a Vault-Hunter, receiving a mission from Hyperion Jack, pre-villain Handsome Jack. Hunted by the Colonel Tungsteena Zarpedon, you end up on the Pandoran moon Elpis, populated by many familiar characters. The Plot is recounted by Gladiator Athena, captured by the Borderlands 2 Vault-Hunters.
The plot acts like an origin story, not only for Handsome Jack, but plenty other characters, since most characters are also present in Borderlands or Borderlands 2.
As it stands, the story is fairly straightforward. There are no major surprises or reveals along the way. Of course, Borderlands was not a title known for its stellar story-telling. It was always more interested in comedy, funny characters and bizarre situations.
Pre-Sequel is not any different. It recounts the tale of Jack’s rise to power, and tries to explain his hatred towards Vault Hunters. You as the player have no agency in the story, everything being recounted, so you should get used to t being led around from point A to point B, and kill everything along the way. This is Borderlands after all.
The ending is sequel-bating, but I think the title itself suggested something like that would happen.
Of course, alongside the main story, you have plenty of side-quests scattered around, most of ‘em plucked from your garden variety MMO. Again, that's part of the course, and there's some fun to be had, while trailing along, trying to kill a certain type-o-lunatic.