Original: Perturbed by Hamlet's continuing deep mourning for his father and his increasingly erratic behavior, Claudius sends for two of Hamlet's acquaintances â "Rosencrantz and Guildensternâ" to find out the cause of Hamlet's changed behavior.
Simplified: When Hamlet's father dies, his uncle Claudius becomes king and marries Hamlet's mother (Gertrude). Hamlet's father appears as a ghost and tells Hamlet that he was murdered by Claudius.