That's right. No "series of trains." Could hardly be simpler than it is. Just a straight journey from Euston Station. And a surprisingly short walk from the Bletchley train station to the entrance to Bletchley Park.
I went last year, and posted about it on this discussion thread in this forum. (My only error in my main post there, was that I referred to my £17 off-peak return (round trip) train ticket as an anytime day return ticket.) The Virgin Trains service from Euston doesn't stop at Bletchley; you ride on to Milton Keynes and have to take a train for the short ride back to Bletchley. I took the Northwestern train which does stop at Bletchley, so no doubling back was required. Off peak travel from Euston (I think it's after 9:30 am) is cheaper than going earlier.
I very much enjoyed Bletchley Park, spent quite a few hours there, including lunch in their cafe, and would like to visit again.