The Project Plan is a somewhat nebulous concept, where if you ask 6 people what it is, you'll get 8 answers.
The consensus seems to land on it being an orienting strategic document that provides direction on managing and controlling the project. It may also include scope, resourcing, governance, and other details. The wise folks at ProJacked built the Project Plan into the guts of the project documentation, and broke it up into logical segments to help to manage all the things that go into managing and controlling the project. The work breakdown structure (WBS) has been hived off in Tasks and Hours.