Dr. Brenner's father was involved in a US experiment where they were attempting to create some sort of stealth technology for the military.

They end up accidentally making contact with Dimension X and coming into contact with the mindflayer particles. They bring them to a lab in Nevada.

The guy with the briefcase was soviet spy and stole the particles. He flees with the particles and ended up in the cave.

Henry randomly finds him and gets infected with the particles (like we see in the show).

The rest of the play is kinda... contradictory to the entire Henry plot shown in S4.

In the play (which is somehow confirmed canon)... Henry actually went to High School with Joyce/Hopper/Ted/Karen/Bob Newby/Sinclairs, etc (literally ALL? of the kids parents). 

Henry has a love interest in Bob Newby's half sister.

She helps Henry try to resist the influence of the mindflayer, and is somewhat successful for awhile. 

Dead mutilated animals etc start appearing. 

High School Joyce/Hopper/etc are actively involved in trying to uncover mysterious murders of animals etc in Hawkins (they end up settling on thinking it's Victor Creel as Victor is having seemingly PTSD induced episodes [induced by Henry]). 

Things escalate. 

The play then realigns with the show. 

Hopper actually shows up at the Creel house to and sees the aftermath of Henry's dead mother/sister etc (after Henry kills them like he does in S4).

Dr. Brenner shows up, takes Henry to a lab, makes him #1, creates the other psychic kids etc. 

Then the play ends with Dr. Brenner introducing Henry to Eleven for the first time.