What does hamlet tell his mother about is trip to England?

Hamlet tells his mother, Gertrude, that he is going to England to visit the King, but this is a lie.

Here's what actually happens:

* Hamlet's true mission: He's actually been sent by Claudius to be executed in England, along with Rosencrantz and Guildenstern. Claudius has written a letter to the King of England requesting Hamlet's death.

* Hamlet's cleverness: Hamlet figures out Claudius's plot and fakes his own death, replacing the execution order with one that orders the death of Rosencrantz and Guildenstern.

* Escape and return: He then returns to Denmark, where he embarks on his final, dramatic confrontation with Claudius.

So, Hamlet's trip to England is a crucial turning point in the play. He manages to escape a certain death and returns to Denmark with a newfound determination to expose Claudius's crimes and seek revenge.