The body count rises, but the fun builds up too, in Innocent Defendant, a South Korean prison drama that has, at heart, the innocence of a family-friendly roller-coaster ride.

A roller coaster, to unpack the metaphor a little, isn't enjoyable because it is unpredictable.

Rather, it is enjoyable precisely because you can see all the tight turns and steep slopes coming, anticipate them and enjoy the heart-stopping, stomach-dropping ride knowing you are fairly safe in a carriage on a fixed track.

So it is with Innocent Defendant, dependable entertainment which swerves from murder mystery to buddy comedy to action thriller, but doesn't go anywhere too upsetting.

In a pretty tricky beginning, the first episode plays like an unremarkable day in the life of a prosecutor, Park Jung Woo (Ji Sung), in a legal drama - a hit-and-run, a funeral where he investigates the accident, his daughter's birthday party - until he wakes up in jail.

He doesn't believe it and has no memory of it, but his cellmates tell him he is on trial for the murders of his wife and daughter. Did he or didn't he do it?

This would have been a real question, covering the show in a cloak of mystery, if the English title hadn't given away the game (Defendant, the title in Korean, leaves more room for doubt). And if the drama hadn't so quickly unveiled the villain, Cha Min Ho (Um Ki Joon), a psychopathic playboy who must be out to get Jung Woo.

Min Ho killed his twin brother and is masquerading as him, just to evade an assault case Jung Woo was bringing against him. Now, Jung Woo has been locked up for crimes he may not have committed, and someone - a loved one who is said to be dead but whose body hasn't been found - might be in real danger because he has forgotten who the enemy is.

This, then, is the crux of the show. It is a thriller that puts Jung Woo at a serious disadvantage, but you can bet he will beat the odds.

As a mystery, it is nowhere as absorbing as Signal, the 2016 crime drama that was a masterclass in the management of knowledge. With that time-bending show, you seldom knew more about the cases than the detectives in the past and present, and it was thrilling to witness new developments, over time but also overnight, with them.

Innocent Defendant is playing a different game. Here, scraps of information - a video that apparently incriminates Jung Woo, seemingly random words that he scrawled on the floor of an isolation cell before he lost his memory - have been scattered among people around him, a mix of unlikely foes and friends.

You know more than him, possibly so you can worry about when he is going to work out whom he can trust and what he can believe. You care too because it's Ji Sung, who plays a grieving family man with despair and tenderness. Won't the show let him get out of jail please?

Another Korean crime drama, Voice, has another man, a cop (Jang Hyuk), grieving over a murdered wife, but it is a different animal.

The show opens with the chilling murder, which is committed as a rookie policewoman (Lee Ha Na) at a call centre listens powerlessly. The would-be victim begs for help as the killer, a man with an unseen face and an unusual crackle in his voice, closes in on her.

It lays the basis for the rest of the drama. Lee, who has a special ability to recognise and remember sounds, teams up with Jang to hunt down the man with the crackly voice. The two also belong to a "golden time" unit that reacts faster to emergency calls.

An early episode, in which they test-drive the unit, is gripping. But as the crimes get gorier and the cops get angrier, the show feels one-note.