It's hard for me to fathom how an actor can start making really good movies, and then start going down the tubes, even though it's clear that there's a lot of energy and enthusiasm left. Perhaps it comes from a failure to be self-critical as one gets popular, or perhaps it's just bad luck. In any event, Martin Lawrence's choice of films are definitely taking a turn for the worse.
In Black Knight, Lawrence plays Jamal Skywalker, a worker at an amusement park designed for a medieval experience who is dragged back to the middle ages in England. There he runs into a petty King (Kevin Conway), his vicious aide (Vincent Regan), and an exotic chambermaid (Marsha Thomason). The latter, who becomes Jamal's love interest, wants him to help her overthrow the King and return the deposed Queen back to the throne.
A lot of the humour in the film is derived from the fact that Jamal is a black person in the midst of many white people, in a medieval setting. This has its moments but most of the gags seem forced. One of the funniest lines in the film is when Jamal says "power to the people" at an execution ordered by the King. This illustrates how much we take for granted in our world today, as well as the level of humour that is present in the film. I recommend giving Black Knight a miss.