This article contains plot/ending details.
The Bass Battle comes from the Guitar Hero game series, where in career mode, the player occasionally faces a famous musician who challenges them to a music battle. It is a pun on the common video game term "Boss Battle".
In the Scott Pilgrim & the Infinite Sadness, the battle starts after Scott hits Todd with his bass, at which point Todd summoned his own bass guitar. Todd started the battle with an "Incredible bass solo", however, as Todd is unable to control his vegan powers, Todd zaps Scott, knocking him back. After being zapped, The Boys!! And Crash! appear to help even the odds, explaining how they can manipulate sound through sheer willpower. They then attempt to use their sound powers launch Scott towards Todd, however Todd's bass power proves too powerful for the band to control, and Todd knocks the band and Scott back. Todd appears to be unbeatable and gloats at his success, but his victory is soon interrupted when the Vegan Police show up, revealing Todd's violations to the Vegan Code. They zap Todd with a Deveganizing Ray, causing him to lose his powers and allowing Scott to physically harm him.
In vs. the World
In Scott Pilgrim vs. the World, the battle starts after Todd threw Scott through a wall into Lee's Palace. Scott starts the battle. Todd begins playing his own bass solo, easily surpassing Scott's skills. Todd's solo overwhelms Scott, and with one powerful blast, knocks Scott through 3 walls, severely damaging Scott's bass in the process. There is also a deleted scene showing a slightly different bass battle but with the same result and Todd remarking to Scott that he knows how to play bass. Todd's bass solo includes the opening rift to Around The World by Red Hot Chili Peppers.
In the Game
In Scott Pilgrim vs. the World: The Game, the Bass Battle starts after the player(s) is chased through the backstage area by Todd. The battle is set up similar the Guitar Hero series, with boards with colored buttons moving down the board. Whilst Todd plays a skillful bass solo, only one note appears on the player's board, pressing any button causes the character to run up to Todd and comically hit him with the bass. Todd, angered by this, flips the player off, then throws the player(s) into the back alley with his vegan powers, where the real fight will begin.