A 25-word summary of the entire play is very difficult. It's basically two to three sentences. To give a more modern comparison, that would be like asking someone to write a 25 word summary of the "Avengers" movie. It can be done; it just won't be really good. The reason is that simply too much happens to too many characters to adequately summarize it ("Avengers" or Hamlet) with 25 words.
Here goes, though.
"Hamlet finds out from his father's ghost that his new step father committed the murder. Hamlet vows revenge. He starts acting crazy in order to find proof. Hamlet has his step father watch a play with a murder in it. Based on Claudius's response, Hamlet finds him guilty. Everybody plots to kill everybody else, and everybody winds up dead by sword or poison."
That's still 63 words. It's not a great summary because it lacks a lot of specifics. If you want it shorter, you could try to use the first two sentences and the last sentence.