There is no story arc to Metallica. It isn't a linear "save the princess" type of game. It's an in-your-face bash fest with multiple modes, mini-wizard modes and a full blown wizard mode where you use strategy to maximize score. Metallica is not for you if you like to play along a predetermined path to finish a "story."
As far as crank it up (CIU) is concerned, the default starts at 15 shots (I believe) to the four "main" shots - crosses, sparky, coffin and snake to unlock CIU. All but the coffin shots can be earned elsewhere on the playfield. Crank it up (CIU) is a mini wizard mode (4 different CIUs) where you are building the jackpot and trying to cash in on it. Building the jackpot is done differently for each of the 4 CIUs. At certain points, you can either take the jackpot, or press your luck to keep increasing it. A great risk/reward here.
In addition, you have the new band member mode (blackened), lady justice mode (ramp combos), seek and destroy mini wizard mode, fuel mode, and graveyard, sparky, coffin and snake multiballs, among other things I'm forgetting.
What I love about Metallica is that you can play it however you want. You can go for CIUs, band member mode, seek and destroy, etc. There is no predetermined path that you need to take. Not being a linear "battle for the kingdom" type game is a strength -- not a weakness -- which is why this game would do well in a lineup of LOTRs, MMs, AFMs, TOTANs, etc.