Hidden Side is a subtheme released on August 1st, 2019. It was unveiled by LEGO in February 14, 2019.  The theme is focused around the haunted town of Newbury and is designed with an augmented reality (AR) app, allowing users to capture ghosts while solving mysteries within the spooky town.
The app is downloadable from the App Store and Google Play. Even though the sets are based around an app, the sets are like any other standard system set and have standard playable features regardless of AR. At launch, the theme had eight sets.
A series of Hidden Side Episodes was released in association with the series. The episodes last about 3 minutes.
- The theme uses unique stylized artwork for the boxes instead of the usual image of the set. The back of the boxes show the physical sets as well as the app feature.
- At the 2019 Toy Fair in London, Hidden Side was shown to visitors in a hidden room under the name "The Hidden".