Look at examples of plot summaries. It is a lot easier to write something if you've looked at a couple examples and get the hang of the kind of wording to use and the way to incorporate all the different elements into one short, cohesive piece.
'J.K. Rowling's "Harry Potter and the Sorcerer's Stone" tells the story of eleven-year-old orphan Harry Potter, who discovers that he is a wizard and goes to study magic at the British school for wizards, Hogwarts. While there he discovers that his parents were killed by the evil wizard, Voldemort, who was destroyed by Harry when he was a baby. With his friends, Ron Weasley, who comes from a large family of wizards, and Hermione Granger, the smartest witch in their year, Harry figures out that the Sorcerer's Stone, which gives eternal life, is hidden on the off-limits third floor. When Harry and his friends discover from Hagrid that the Sorcerer's Stone is no longer be safe, they go to stop the thief themselves, who they think is Professor Snape, who hates Harry. When Harry finds the Stone, he discovers that the thief is Professor Quirrell, who is possessed by Voldemort. Because of a spell cast by Harry's mother, he is able to defeat Quirrell and Voldemort is forced back into hiding.'
'Homer's epic poem "The Odyssey" tells the story of the Greek hero, Odysseus, and ten-year voyage to get home to the island of Ithaca where his wife, Penelope, and his son, Telemachus waited. It begins with Odysseus imprisoned by the nymph Calypso until the Greek Gods force her to free him. The god Poseidon, who harbors a grudge against Odysseus for blinding his son the Cyclops Polyphemus previously in his travels, attempts to wreck his ship, but is stopped by the goddess Athena. Odysseus makes it to Scheria, home of the Phaeacians, where he is given safe passage and asked about his journeys to this point. Odysseus tells them of the variety of adventures he suffered through with his crew, the trip to the Land of the Lotus Eaters, his blinding of Polyphemus, his love affair with the witch-goddess Circe, the deadly Sirens, the journey into Hades, and his fight with the sea monster Scylla among them. The Phaeacians take him safely to Ithaca, where he enters the hall disguised as a beggar. In Ithaca, supposing Odysseus to be dead, suitors have taken over his hall, tried to kill his son and tried to convince Penelope to choose one of them. Penelope, believing Odysseus to be alive, has refused. She arranges a contest with Odysseus's bow, that only he can string. Once he's strung it, he shoots all the suitors and is reunited with his family.'
These summaries cover the main plots of the stories that they're summarizing. They use sentences like "When Harry finds the Stone..." instead of explaining exactly what it took to find the stone, which is not the point of a summary. They are brief and they focus only on the most important main characters, like Odysseus, Penelope, the gods, etc.