Not necessarily. Otherwise it would be impossible to prosecute someone for meticulously planning a terrorist action, who is only stopped when they are just about to purchase the materiel required to carry out the act. No action has tajen place - actual terrorism has not occurred, and there are no physical tools or similar present. But the intent (or Mens Rea, guilty mind) is there, so you can be prosecuted.
And, of coure, there are crimes of "Conspiracy to X" that involve merely the intent to commit a crime.
A crime requires the confluence of the required mens rea and the required actus reus; the former alone is not sufficient.
For the kind of plot you describe, prosecution would usually occur when there are multiple persons involved based on conspiracy charges, which require an overt act in furtherance of the conspiracy (which can be a fairly minor act, but it still requires an act.)
Mens rea is a required component of many crimes, yes, however it is not sufficient alone to be criminal. Research with malicious intent is not a crime. What you describe is not criminal.
And yet, people in the UK have certainly been prosecuted for looking up jihadi websites with the intent to download bomb-making instructions... Although, the actual crime may be posession of said information? It's hard to tell, particularly living in a country where 'glorifying terrorism' is now a crime - http://en.wikipedia.org/wiki/Terrorism_Act_2006 - as is 'encouraging terrorism' too.
I bet the prosecutors are kicking themselves for not thinking of that one earlier, as well - they could have simply rocked up to certain pubs in Belfast and arrested and jailed everone singing IRA songs ;)
And, of coure, there are crimes of "Conspiracy to X" that involve merely the intent to commit a crime.