In Flight review: Katherine Kelly stars in a plot-laden thriller which feels like trying to walk up the down escalator

Every time you thought you'd finally fought your way to the top, another step appeared from the plot room down below and the whole trudge started again.

That's not to say you didn't appreciate the work-out this twisty-turny little thriller gave your faculties, it's just that at times it felt like you weren't getting anywhere.

Katherine Kelly stars as Jo Conran, a flight attendant with a messy personal life.

Most protagonists in dramas have messy personal lives these days, it's just that Jo's seems a particular tangle.

Having fallen out with her teenage son Sonny (Harry Cadby) over his relationship with his dad, and her ex-partner, she has to step up when his travelling overseas goes horrendously pear-shaped and he winds up in jail for killing a man in a Bulgarian bar fight.

Then there's her affair with married customs officer Dom (Ashley Thomas) – he's gone back to his wife and kids, while she still pines after him.

Add in turning a blind eye to a flight crew colleague's layover, well, layovers and Jo has a lot on her plate.

But wait, here comes another one of those pesky escalator steps.

She's approached by Cormac – who may have an attractively soft Northern Irish and a very well-tended beard, but is clearly a bad danger – who threatens Sonny's well-being in prison if Jo doesn't agree to smuggle drugs into the UK from her various airline destinations.

All the while, you keep plodding your way up the escalator.

Jo tells Dom about the smuggling, and he helps her get through bag checks, putting him and his family at risk, both from the law and the bad guys.

Cormac is threatened by unknown forces, leading you to wonder if he's just a patsy too.

The man Sonny's accused of killing turns out to be the son of a notorious gang boss in Sofia - then there's a pregnant girlfriend, a suspicious wife and a light-fingered airport cleaner.

Those steps keep coming and by the end of episode three you feel as frazzled as Jo heading through Bangkok customs with a bag of heroin taped to her stomach.

But you wonder if those steps will ever stop revolving round and round and if you ever will reach the end of the climbing.

And In Flight starts to teeter under the weight of the plot, with some sequences extended over what seems like weeks in the timeline – particularly when Sonny finds himself in hospital after a prison cell shivving – and others when apparently of weeks of cabin luggage drug-smuggling are compressed into a 30 second-long montage.

Jo is clearly a woman of some resolve, and that steeliness only increases as she fights to protect her son as best she can.

A moment when she comes over all John McClane, stripping to her vest before manhandling her son's hospital bed around dimly-lit corridors, is in danger of bringing the escalator to a grinding halt, but such is Kelly's sinewy performance that they get away with it.

It has the air of a Channel 5 drama of the week, which would normally star Sheridan Smith and end in a welter of plot holes and silliness, but In Flight muddies the characters' motivations convincingly, never allowing you to think they're all wholly good or wholly rotten.

In the end, all the characters are stuck trying to get up that down escalator, some will fall backwards, some might eventually got to the top and get away.

As drug mule wrangler Cormac says: “This is your life now. Make peace with it, it's the easiest way.”

In the meantime, us viewers will keep putting our steps in, and hope it pays off when we finally get to the top.