A rollercoaster this week, but in the worst possible sense of the word as a dull start gives way to a brilliantly promising central act before stuttering to a sedate and underwhelming halt.

There’s every appearance of a monster-of-the-week job to begin with, as a whole train carriage of commuters die (every trace of biological energy instantly drained from their bodies) on the appearance of a mysterious stranger (who looks not unlike Peter Weller out of ‘Robocop’). As the team move to track the stranger down, Walter continues to procrastinate on whether to tell Peter the big secret of his heritage.

Things take a turn for the awesome at the half-way point when we find out that the stranger is a Dr Alistair Peck – a genius of epic proportions who has uncovered the secret of time travel (encoded into cybernetic components that he has hand-forged and then – delightfully – surgically implanted under his own skin).

For a moment it looks as though ‘Fringe’ is going to take one of its sudden turns into the genuinely good: in the middle of a raid Peck suddenly leaps back through time to the train car (it turns out that his arrival sucks the energy out of anything in the vicinity – computer, vehicle or living thing).

We then once again run through the events leading up to his identification and near-apprehension. This time Walter sets out to reason with Peck, whose motive for time-travel is to rescue his fiancee who died in a car crash.

Someone has let John Noble off the leash recently as his performance in this sequence is genuinely powerful stuff – he really is the absolute core of this show, especially this season.

Walter’s plans go awry (but not before revealing to Peck that he is awaiting a sign from God – in the form of a white tulip -  before telling Peter of his extra-dimensional origins) and Peck makes the leap back to the day of his wife’s death… and this is where it all goes a bit floppy.

The pre-climactic confrontation between the two scientists makes a big deal of not knowing how actions (whether in terms of time-travel or multi-dimensional jaunts) effect the universe. The greedy viewer will be rubbing their hands together expecting a ‘Butterfly Effect’ impact on the show’s dynamic – will Peck’s actions perhaps bring a character back from the dead, or introduce some-one new a la Dawn from ‘Buffy’? Sadly neither of these, nor anything else of consequence: Peck manages to rescue his wife – and they promptly get mashed by another car.

The ‘Time Machine’ remake did this ‘inevitability of fate’ thing, but there are two aspects to ‘Fringe’s approach that really stick in the throat. Firstly the theme of the episode hadn’t been the inevitability of fate, but rather the unforeseen changes that can be wrought by irresponsible science pursued with good intentions: as a result the ending felt like a huge cop-out.

Secondly this ending left so many questions unanswered as to be nonsensical – key among these being that future-Peck travelled back in time and died with his wife of the past, so what happened to past-Peck? A savvy viewer may slyly hope that he will re-emerge in a future episode.

However it’s pretty unequivocally stated by a third party that Peck is dead – so unless he’s gone into hiding for some reason that ain’t going to happen. Which makes the whole thing feel a bit pointless and, as with last week, the lasting impact is questionable, the only effect to carry over likely to be a letter that Peck arranged, ‘Back to the Future III’ style, to be delivered to Walter Bishop, containing a drawing of a white tulip.

This episode can’t help but feel like a wasted chance, especially as ‘Fringe’ does such quantum shifts very well. At its worst this could be a worrying portent of a show unwilling to take risks. Given that that is the only thing keeping ‘Fringe’ from fairly generic sci-fi mediocrity it’s something to be concerned about.