To complete the Sneaking Mission Challenge in Lego Star Wars: The Skywalker Saga, you need to discover the underground route into the hangar bay during the First Order of Business level in Episode VII – The Force Awakens. You can complete this Challenge on your first playthrough on Story mode, using the default characters, Finn (FN-2187) and Poe Dameron (Jacket).
The objective markers will take you through the overground route, so you want to ignore them right off the bat. Turn right at the start of the level, then go through the door on the left. Use Finn (FN-2187) to throw a grenade at the raised platform, then build a door handle from the debris, and switch to Poe Dameron (Jacket) to pull the handle and open the door. Go through the door and up the slope then, in the next room, you need to put the green, purple, and orange energy cells in the correct slots under the screen on the south wall.
The green energy cell is in the southeast corner of the room, the purple energy cell by the window in the north wall, and the orange energy cell in the southwest corner of the room. Completing this puzzle opens the door in the east wall. Go through that door, follow the corridor, then drop down the gap. You’re now in the underground route, but you need to pass all the way through it to complete the Challenge.
Move the large box so that it blocks the steam, then continue along the tunnel and block some more steam with another box. You’ll need to smash an obstacle and use the second box again to block a third set of steam jets. Pull the lever to turn off the steam, then use Poe Dameron (Jacket) to open the hatch in the ceiling. Climb the ladder to reach the hangar bay and complete the Sneaking Mission Challenge.
