Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
Rick and Morty is a wonderfully insane animated science-fiction series for adults about Rick, a nihilistic mad scientist (the smartest man on earth) and Morty, his anxious 14-year-old grandson, and his family and their misadventures across the galaxy and throughout other dimensions. Rick can make a device to do whatever he wants, and he does, including a spaceship car to travel to other planets with a gun he made that can make a portal that lets him travel to other places and dimensions.
In this episode, Rick's space car has a flat tire and he gets out to fix it. (This is an absurd comedy, so you need to accept all sorts of impossible and ridiculous things.) Morty gets out and is immediately bitten by a snake wearing an astronaut suit. The snake's fangs punched through its helmet and poisoned Morty. Morty kills the snake with a hubcap. Then venom goes to Morty's head and nearly kills him, so Rick flies them to the snake planet where his computer can do an analysis so he can make an anti-venom. He makes it then injects it into Morty, who is cured instantly.
Rick discovers that the snake planet is inhabited by 19 billion snakes who have developed a complex society that is on the brink of global war over race. He laughs at the idea of racist snakes. He turns on a snake radio station and hears "snake jazz" which is just a rhythmic hissing. Morty brings some snake jazz back to Earth, and later his sister and her girlfriend rock out to snake jazz in her bedroom.
Feeling bad about killing it, Morty wants to return the dead astronaut snake to its planet but Rick refuses. After they return home, Morty goes to a pet store to buy a snake that looks like it so he put it back on the planet. He asks the salesman if a snake that resembles the astro-snake is friendly. The salesman says dryly "It's a snake" which makes sense to anyone who has ever had a pet snake. Morty paints the snake to look like the dead astro-snake (named Buzz Advil.) The new snake bites Morty repeatedly before he stuffs it into the snake astronaut suit. Morty steals Rick's space ship and flies back to the snake planet, where he releases the new snake in the atmosphere in a parachute. The snake is rescued and hailed as a hero, but then the snake leaders sense something is wrong and examine the new snake. We don't know exactly what is happening because all we hear when the snakes communicate is hissing. (I appreciate that the animators did not make the snake language intelligible to human ears. Their written language is also unintelligible. It's just snake-like squiggles. We even see a ship in the astro-snake rescue party named the "S.S.SSSS.") The snake leaders bring in a snake professor to examine the new snake, but the new snake grabs him and mates with him.
Then it gets very weird and hard to understand, with all sorts of time traveling snakes and Ricks and Mortys in various timelines, and a whole parody of The Terminator movies with a robot snake from the resistance that time-travels to the past to kill other snakes, and then more robots time-travel back to kill them. (This episode is actually a scathing parody of the problems with time travel stories and all of their inconsistencies.) Rick tells Morty that by bringing the new snake to their planet, he gave the snakes proof that there was something bigger and scarier to unite against beside race, and so they progressed into the future developing robots and time travel. Then, as the robots took over, they went to war with the snakes (like what happened in The Terminator).
Rick and Morty travel back to the snake planet to try to fix things. They start killing snakes and robots to get to the heart of SnakeNet, where they discover the snakes haven't yet developed time travel. They want them to have time travel, so they dress in snake costumes and go back in time to "Snake M.I.T." in the snake 1980s, where they leave a book that explains how to invent time travel. And they do. The snake students use it to go back in time to save snake Abraham Lincoln from getting assassinated and to kill Snake Hitler. Then everthing turns into a time-travel nightmare with so many different travelers from so many times.
Finally, Rick and Morty return home to discover that the snakes have overtaken Earth and Morty's sister and mother are fighting them off on the front lawn. Then the Universe Time Police become aware of the snakes' time crimes. They go back in time to stop the snakes from developing all tools, so they never develop time travel or space travel and probably any other technologies, leaving them like normal snakes that do little more than hunt, eat, and breed. Then all of the snakes that came to Earth disappear. Snakes have no hands or limbs to use to invent technology or have baseball teams or jet pilots, so they're better off going back to crawling in the dirt. It has worked for them on this planet for more than a hundred million years, so why try to fix what ain't broke.