Boxes Locked! Keys, Please! is the 3rd level in Splatoon 3's story mode, Return of the Mammalians. It introduces the keys and vaults feature into the game.
First Checkpoint[edit]
When you start, you see a large amount of crates in front of you. Break them all, and move forward. You will see a vault with an ink rail in it. Go around it and break the box. Grab the key inside, and open the vault by standing near it. Use the ink rail to swim across.
Jump off of the ink rail and you'll reach the Final Checkpoint.
Final Checkpoint[edit]
You will see a Splat-Switch in front of you once you reach the final checkpoint, after inking the Splat-Switch, it will reveal a vault within a vault. You will need two keys to unlock these vaults, and these keys can be obtained on the two side platforms to the left, and to the right of the main vault platform. Access to these islands is through Inkrails.
Left Platform[edit]
Ink the Inkrail on the left to swim across to the left platform. The left platform has a key located in one of the corners which will unlock a vault concealing a Inkrail. This inkrail crosses the main platform, which is important for later.
Right Platform[edit]
Throw little buddy at the Inkrail in the distance on right platform which will allow you to cross to the right platform. Ink the Inkrail below the tower on the right platform to climb up the tower. Once at the top, break the crate containing a key. Throw little buddy at the top of the tower at a Inkrail near the floating power eggs, which will activate a Inkrail crossing the main platform. Now there are Inkrails coming from both platforms.
Main Platform[edit]
Swim back to the main platform now that both of the platforms' Inkrails are activated. Jump from the lower Inkrail to the upper Inkrail, and land on top of the cube containing the vault. Once atop the cube, there's a pile of crates that you break, one will contain a key. You should have two keys, which you will use inside the cube containing the vault within the vault.
Once the vault within the vault on the main platform is unlocked, ink the Inkrail and swim to the Zapfish. Grab the Zapfish and complete the level.