Here’s a rough summary:
Ep1: [spoiler]Owen Milgrim, Jonah Hill’s character is introduced. He’s the black sheep of his rich family. He’s lonely and mentally ill and loses his job and enrolls in an experimental drug trial. He repeatedly sees Emma Stone’s character Annie Landsberg in ads and other places, then sees her at the drug trial location as well.
The world of the show is also shown, it’s a retro-future alternative universe 2018, where there doesn’t seem to be the internet or cell phones, but there are ubiquitous ads, including actual humans who will read them to you on the subway if you can’t pay the fare, there are poop cleaner robots on the streets, and other things slightly different than our world. [/spoiler]
Ep2: Emma Stone’s character Annie Landsberg is more fully introduced, she’s addicted to the experimental pills that are in the drug trial, she wants to quit and go to Salt Lake City to see her sister, but ends up blackmailing her way into the study. At the end of the episode she takes the A pill and then we see her on a road trip with her sister that ends with a terrible car accident.
Ep3: [spoiler] Dr. Muramoto who is running the study notices irregulatries in the data from Annie and Owen, since Annie is already addicted to the pill and Owen didn’t take his because of his delusions he thought he needed to stay awake and ready for an assignment. Muramoto questions them both and dies while questioning Annie. This leads to Dr. Azumi Fujita going to go get Dr. James K. Mantleray back to run the study, although he had previously been pushed out of the company.
The sentient computer GRTA is sad about Muramoto’s death, and her crying leads to the wires connecting Annie and Owen’s seats to seemingly be soldered together.
Ep4: Annie and Owen have taken the B pill, and they find themselves in a caper in Long Island to retrieve a lemur. The new reality is unexplained, but they seem to be long married, but there are things from the previous episodes that bleed over, like the big truck that hit Annie’s car. They are also both participating, it’s not like it’s a dream that Owen is having and dreaming that Annie is in, it is a shared dream/hallucination/reflection.
Ep5: Now Annie and Owen are grifters at a seance in the Neberdine mansion in the 1940s. The scientists have realized that Annie and Owen are sharing dreams and keep trying to zap Annie out of Owen’s grifting adventure but she keeps popping back in.
If you have any more specific questions, I can try to answer them. There’s a lot of callbacks and things that might not be necessary to catch but I think makes things better. It’s definitely a show that you need to be paying attention to while watching.