Elementary Season 4 Episode 15
Up to Heaven and Down to Hell
Joan and Sherlock assist the NYPD in investigating when a wealthy old woman falls to her death, inadvertantly crushing a man on the sidewalk below. It quickly becomes apparent that she was murdered. At first, they can't find a firm motive as her estate was going to be tied up in legal challenges for years, until Sherlock discovers that developer William Hull was after the air rights above her building.
Hull wanted to build a skyscaper nearby. Someone at his architect's company was feeding information to protesters. The lead protester was murdered in a burglary. Sherlock suspects a connection.
Using files purloined from the architecture firm, Sherlock discovers that the building was fundamentally flawed. In order to save face, the architect needed a way to make the building shorter without taking responsibility. Killing the old woman would tie up her estate (and its air rights) for years, thus providing him with the perfect excuse to make the building shorter.
The protester was killed because he stumbled on the architect stealing back his stolen files.
Meanwhile, Joan learns that Gregson is in a relationship with a former cop, Paige. Paige eventually reveals that she has been diagnosed with MS. Gregson decides to stay with her despite this.