Merlin (BBC One, 6.15pm) It simply wouldn’t be Saturday night without a family-friendly journey into fantasy, and Merlin is certainly casting a spell over audiences each weekend.

Tonight the action hots up with a storyline as full of myth, magic, murder and mystery as we have ever come to expect over the past few weeks.

Uther is delighted when Lady Catrina arrives in Camelot, in the first episode of an exciting two-parter in the magical fantasy drama. Merlin and Gaius, however, suspect that she is not what she seems.

Uther is captivated by his latest guest but Arthur is appalled by his father’s flirting. Merlin is horrified for a different reason — he discovers that Lady Catrina is really a troll but can’t get Arthur to believe him.

Catrina and her strange servant, Jonas, are welcomed into the heart of Camelot and it is up to Merlin to prove that she is a monster and to save Uther from ruin.

Murderland (Monday, ITV, 9pm) Some television shows are getting too clever for their own good. You could be forgiven for thinking this of Murderland, but rest assured that it is complex, not confusing and plays out very nicely indeed.

Furthermore, it boasts the considerable acting talents of Robbie Coltrane, who puts in a must-see performance to supersede any of his previous works.

The second part begins in the present day as Carol persuades Hain to return to London with her to reinvestigate her mother’s murder — paying particular attention to the original suspects and the massage parlour in which she worked.

As the detective relives the night of the killing, his perspective as the policeman who was in charge of the case reveals another side to the story and uncovers new clues that Carol failed to pick up on as a teenager.

Be sure to tune in next week when the story comes to a climactic close.

The Force (Tuesday, Channel 4, 9pm) There are some crimes so heinous it was once widely believed that they only really ever took place in the inner cities. However, offences such as murder, rape and arson now have to be investigated by police forces across the UK.

The three-part series following the realities of modern detective work undertaken by the officers of the Hampshire Constabulary comes to a close with this final edition revealing how, in September 2007, someone maliciously set fire to a block of flats in Portsmouth, claiming the life of resident Kevin Holmes.

Nine months later and the murder remains unsolved as the police have found nothing to indicate why anyone would want to kill him. It’s a tough case of the usually efficient Hampshire Major Crime Unit, which has one of the lowest unsolved crime rates in the country.

With a fresh look required, the case is handed to an experienced Detective Chief Inspector — the third senior officer to take charge. Can he find the person responsible?