Crime Traveller was a very shortlived (one season, consisting of eight 50-minute episodes - Firefly had it good by comparison) science fiction/detective series broadcast in 1997 on BBC1.
I'll come out and say it right at the start: It was not the greatest series ever made. Whilst obviously in any series dealing with time travel one has to allow for certain liberties to be taken with the known laws of physics, the mechanics of exactly how time travel in this series works - or more importantly, how one returns to one's own time - is logistically a bit of a mess (I'll get onto that in a moment). If one buys a lottery ticket in the past and brings into the present, it will wipe itself clean, because "the future doesn't exist"; or something along those lines, which I don't think really makes sense. And I'm no expert on the workings of police stations, but I'm pretty sure no police station in the UK functions in quite the same way as that shown in this series (for one thing, the protagonist Jeff Slade is so rude to his boss on so many occasions that he'd have probably been fired ages ago; for another...do police stations actually have "science officers"? I don't think so, but am happy to be proven wrong; for another, Morris is so terminally thick that I doubt in real life he'd have even got a job in the police, much less been able to keep it).
However, I come to praise Crime Traveller, not to slag it off. So, despite my criticisms, the other thing I'll say is - go into it with an open mind. Because, if you've heard anything about it at all, it's probably something negative. And it's probably something from the - usually reliable - SFX magazine, which published a review - by former Doctor Who writer and creator of Star Cops, Chris Boucher - of the show, which was extremely scathing, unfairly so in my humble opinion. When, a few years later, SFX ran a regular series of articles, "Isn't it about time you gave [xxx] another chance," which would features arguments in defence of much-maligned TV programmes, films and books, the article on Crime Traveller consisted of a two page spread featuring the single word "NO!" in a font as large as possible. 17 years later, it still hasn't seen fit to forgive the programme, as evidenced by its comments in its recent time travel special edition.
Which is...a shame. As I said, it's not a perfect programme, but I've seen far worse. Its main crime, as far as I can tell, was (ironically, for a programme about time travel) an accident of timing - it first went out about a year after the 1996 Doctor Who TV Movie, the one starring Paul McGann which everybody thought was going to herald a new dawn for the nation's favourite Time Lord. So when we expected by rights to be devouring a new, albeit Vancouver-based, series of TARDIS travels, we were actually served up a new creation starring the guy who used to play David Wicks in EastEnders and the Kochanski who wasn't Clare Grogan in Red Dwarf. Some disappointment was inevitable, but the level of vitriol the show has been subjected to by Certain People seems a bit out of proportion.
Perhaps an explanation of exactly what the show entails would be in order. Jeff Slade (Michael French) is a detective working in a local (rather anonymous, although some of the series was filmed in Reading) police station. He discovers that his colleague Holly Turner (Chloë Annett) has a working time machine, thanks to her father, and together they use it to solve crimes. The time machine can only send them back a limited amount of time, and the amount of time is entirely random (IIRC the longest is only a couple of days). Once in the past (they can't travel into the future), Jeff and Holly become a part of the events of the past (leading to some pleasantly intricate timey-wimey plots). (TV Tropes refers to this as "Stable Time Loop," and I'll be honest, it's my favourite time travel trope.)
Now...the machine is in a room in Holly Turner's flat (trivia: the building used for the flats would also be used as the location for Maddie's flat in Jonathan Creek). When the machine has deposited the traveller back in the past, out of it pops a little electronic device which the traveller wears on his or her wrist, like a watch. The traveller must return to the machine by the time he/she travelled back from, and place the electronic gadget back in its slot in the machine. The gadget helpfully bleeps, with increasing frequency, to warn the traveller that his time is nearly up. If you fail to get back to the machine in time, you will be trapped in "the loop of infinity".
Now...there are obvious logistical difficulties with this set up, most jarring of which is - why don't the travellers encounter themselves about to set off on their travels when they return to the machine? Mostly, this is just glossed over, although on at least one occasion we see the effects of the travellers' activities during the return to the machine, as they're about to set off (we only understand what's going on later...). In a later episode, this is botched horribly, as on the return to the machine (or at least, a machine - by this time they've found another one that is being worked on by someone else) the building is in a state of maximum alert with fires and alarms going off everywhere - none of which we saw when Jeff initially set off to travel back in time.
Nevertheless, that problem aside, the show's creator and sole writer Anthony Horowitz (better known for creating Foyle's War and writing the Alex Rider series of books) crafted some intricate time travel plots for the series (my personal favourites are episode 3, Fashion Shoot, episode 4, The Revenge of the Chronology Protection Hypothesis, episode 5, Sins of the Father, and episode 7, The Lottery Experiment) and it's a shame it wasn't given a second series to enable it to really find its feet. Although the character of Morris (as mentioned previously) is rather too dimwitted to be taken credibly, the other regular supporting characters, including Nicky (played by Richard Dempsey, whom genre fans may recognise from his role as Peter Pevensie in the BBC's late '80s Narnia adaptations) and Slade's boss, Grisham (Sue Johnston, best known for Brookside and The Royle Family) are well written and acted. There's also a wonderfully scenery-chewing guest appearance by Stephen Greif (Blake's 7's first Travis and Citizen Smith's Harry Fenning) in the episode Sins of the Father.
The series is well worth the under a fiver it will cost you to buy the complete DVD boxed set from Amazon marketplace, and the under seven hours it will take you to watch it. On the negative side, there isn't much of an actual fandom for the show - there is a fanpage dedicated to it, which even includes some fanfic (ten stories, written by about four authors in total - and I've no idea how many, if any, of them are still active in the fandom), but if intricate, enjoyable time travel plots are your thing, with no angsty baggage and only the slightest hint that Jeff and Holly may be ship material, then it's well worth checking the series out. Come on in - the water's lovely!