Marvel Super Heroes — Disney Infinity:

The "good piece" you're referring to is likely the piece . This translucent blue sculpt of Avengers Tower is the core component that unlocks the primary story campaign in Disney Infinity: Marvel Super Heroes (2.0 Edition) . Essential Marvel Play Sets

: A separate piece featuring a Spider-Man symbol being consumed by the Venom symbiote. It allows you to play through the Manhattan-based Spider-Man storyline. Disney Infinity: Marvel Super Heroes

Each "piece" acts as a physical key to unlock a specific digital world with its own missions and story. The "good piece" you're referring to is likely the piece