In Luigi’s Mansion 2, you cannot leave a mission anytime you want. The game is structured in a way that you must complete each mission before progressing to the next one. Here’s a detailed explanation:

1. Mission Structure: The game is divided into various mansions, each with its own set of missions. These missions are typically marked with letters and are designed to guide you through the mansion, helping you explore and collect items, including gems and Boos.

2. Progression: To progress through the game, you must complete each mission in sequence. You cannot skip ahead to a later mission without completing the previous one. This ensures that you have the necessary tools and knowledge to tackle the challenges that come later in the game.

3. Tools and Upgrades: Each mission often introduces new tools or upgrades for your Poltergust 5000, which are essential for accessing certain areas or solving puzzles. For example, the Stroboscope is introduced early in the game and is crucial for finding some of the gems.

4. Gem Collection: The gems are scattered throughout each mansion and are often hidden in hard-to-reach places. You need to complete the missions to unlock new areas and gain access to these hidden gems.

5. Boo Collection: Similarly, collecting all the Boos in each mansion is a requirement for unlocking special missions and rewards. These missions provide additional challenges and rewards but are only accessible after completing the main missions.

6. Professor E. Gadd’s Vault: The ultimate goal is to collect all the gems in each mansion to unlock a statue in Professor E. Gadd’s vault. This statue is a reward for your efforts and serves as a completionist’s badge.

In summary, while you can explore the mansions and interact with various objects and characters, you cannot leave a mission in Luigi’s Mansion 2 until you have completed it. The game’s design ensures that you progress through the story and challenges in a linear fashion, providing a structured experience for players.