The Inlaws is a comedy adventure game that is designed with elements from sitcoms such as audience laughter. The game consists of a single episode and the player takes control of a middle aged man called Ed who, along with his family, visits his in-laws to celebrate the tenth birthday of the son in the family. Ed is a mischievous being, though, who can't resist making the life of his poor brother-in-law as miserable as possible. There are thus two objectives: mess with the son's birthday gifts and ruin the performance of the hired entertainment troupe.
The player controls Ed from a third-person perspective and can explore a three dimensional environment that consists of the inlaws' house as well as their back garden. The gameplay is typical for an adventure game as Ed collects items that are stored in his inventory where they can be combined with others to create new ones. By using the items on certain objects in environment or on a character, cut-scenes are triggered that advance the story. Some characters can be talked to and the player then has to choose the dialogue option that will please the character. For each puzzle solved the player earns points. When stuck it is possible to use a help option which shows the direction to the object or character that the player needs to interact with next. When using it the player will not earn any points though.