Gearbox and 2k's Borderlands: The Pre-Sequel is a decent game and a fine entry in the series, with plenty of guns and fun characters. However, the story is totally unnecessary to the franchise overall.
Borderlands: The Pre-Sequel is set between the first and second game, and it attempts to detail how beloved villain Handsome Jack from Borderlands 2 became the power mad, evil Hyperion leader. Borderlands 2 goes out of its way several times to show how petty and cruel Jack is, in an effort to craft a character with so much charisma and snark that players both love him and absolutely hate him.
Having a story detail this villain's backstory could have been interesting, but the result is subpar at best. The Pre-Sequel almost makes the megalomaniacal murderer a sympathetic character, and after the events that transpire in Borderlands 2, it just reminds people of his horrible acts.
The game's story also suffers from an overall lack of good content. The big reveal in the game is that Jack's efforts to find a vault on the moon get sabotaged by Borderlands veteran characters Moxxi, Lilith and Roland. After Jack finally gets to the vault, it scars him and he is driven insane. The entire plot can be summarized in just a few sentences, so aside from a few unique gameplay mechanics, Borderlands: The Pre-Sequel is mostly pointless.
The game also doesn't do much for the playable characters, either. The main vault hunters in the game are Nisha, Wilhelm, Athena and Claptrap, but of the four, only Nisha and Wilhelm go on to do anything important later. In Borderlands 2, it's revealed that Nisha becomes the Sheriff of Lynchwood on the planet Pandora and that Wilhelm becomes the enforcer of Jack's army, but there's no information about their lives before. Rather than expanding upon characters players may see later, The Pre-Sequel just brings them along for the ride.
Meanwhile, the Lost Legion attacks and forces the player to retreat to the moon at the beginning of the game, and the group is built up as a big force of abandoned miners who have taken to guarding the vault. Their leader serves as a boss near the end of the game, but in the grand scheme of the story of Borderlands, this is the only time the Legion is ever mentioned -- so the threat can't possibly be that bad, in the grand scheme of things.
Borderlands: The Pre-Sequel tries to tie the first two games together by detailing the rise of the second title's villain. Though it's an interesting idea, the story is ultimately unimportant and unnecessary.