Where was the "catastrophe" of this play found? act i act ii, scene i act iii, scene iii act v hamlet

In the play "Hamlet" by William Shakespeare the catastrophe was found in act v.  In this final act Hamlet finally killed Claudius. However, it did not happen as was expected in a revenge play. He did it in an improvised way that is the reason why, even though it was a murder, it was not seen as the ordinary revenge story.