The Doomsday Contract, also known as Shylock, was an unproduced serial due to be part of season 17. John Lloyd wrote the story using elements from his unproduced novel GiGax.
It would feature the Fourth Doctor saving Earth from a corporation that wished to buy it to receive a matter-transmutation device. Lloyd was asked to rewrite the script a few times. He eventually gave up to work on Not the Nine O'Clock News. Douglas Adams, with Lloyd's permission, allowed Allan Prior to rewrite it, but his script was rejected. Adams gave the project to another author to be part of season 18, but Adams soon left the series and the serial was ultimately cancelled.[1]
An audio adaptation was released as part of The Lost Stories range by Big Finish Productions in March 2021.