What is a postmodern nonlinear storyline?

Answer 1

Watch "Memento" with Guy Pierce

Answer 2

Postmodern: the historical period that began roughly after world war II and continues to this day (tho' it may be ending--we won't know, of course, until a while after it's gone).

Nonlinear: a linear story line tells the tale in a straightforward, cause and effect manner. Most story telling uses a linear structure (some, like Oedipus Rex or Death of a Salesman, use a "late point of attack", beginning the story near the end--but they incorporate logical flashbacks and recollections in the telling so that the backstory is covered.

The most non-linear plots would have been those of the Dadaists and Absurdists, back in the modern period. I guess you could consider Beckett a postmodernist? A more solid couple of postmodern playwrights in the nonlinear style would be Kopit and Albee, and oh yes there were those Antony Newley musicals "Stop the World I Want to Get Off" and "The Roar of the Greasepaint, the Smell of the Crowd". I recall seeing a sort of experimental show called "A Rat's Mass" that was nonlinear, back around 1971...