It's not actually _that_ nonlinear - follows philosophy on time travel through Aristotle, then Godel & Einstein and then through to Hawking.

That said, the nonlinear part: picks out every now and again a pop culture echo of the current theory being discussed, like the Terminator or the Time Machine or Heinlein's "—All You Zombies—" i.e. the one where a person is their own mother, father, son and daughter. (I really wish it talked about Looper or 11/22/63 or Rick & Morty.)

Discusses whether time travel is logically possible, how paradoxes could ever work, causality - the overall tone being time travel is basically impossible as we know it but boy is it ever fun to think about.