There is a secret alternate ending in Sly Cooper: Thieves in Time that you’re going to have to work extra hard to unlock.
In order to see the hidden alternate ending, you have to get the Platinum trophy, which requires you to unlock all of the other trophies in the game.
Once you get the Platinum trophy, you’ll have to beat the game one more time, and then you’ll see the secret alternate ending, aka “best” ending.
Have fun, you’ve got some work ahead of you!
