You don't need to be a sleuth to figure out why Agatha Christie set so many of her crime novels in Devon. She was born in Torquay, fell in love there more than once and spent the happiest years of her life in a holiday home high above the River Dart with her second husband.

Even so, there are a few intriguing mysteries afoot at Greenway, the beautiful house which the National Trust has recently opened to the public: like what the 12-sided box with a keyhole but no key, which sits on a chest in the author's old bedroom, is all about; not to mention the grinning skull with a frog perched on its head which crouches on a hall sideboard. They are two of many strange but fascinating objects in the home Christie shared with Max Mallowan, the toyboy archaeologist she met when, as a 40-year-old divorcee, she went out to Iraq alone on the Orient-Express to visit his dig. 

The guide at Greenway jokes that perhaps the blindfolded cherub on the hall light fitting is an allusion to love being blind, as Mallowan cut a dashing figure beside the already matronly Christie.

The couple bought Greenway just before war broke out, and the grand pile, first owned by Walter Raleigh's ancestors, was requisitioned by the Americans. When the soldiers left, they offered to leave a row of 13 latrines - tempting, you might think, to a woman who wrote of the lavatory as"the perfect place for quiet meditation." They were removed, though her own loo, resplendent with mahogany seat, remained one of her favourite rooms.

She also loved the drawing room, and a picture shows her painting the mantelpiece her favourite colour, cream. That matronly figure may have been a result of the fact that the author liked to drink the main ingredient of Devon's famous cream teas by the cup, as well as paint her walls that shade. With the 120th anniversary of Christie's birth coming up (and, coincidentally, the 80th and 90th of, respectively, Miss Marple and Hercule Poirot), this is a great time to go poking around not only Greenway, but the exquisite coastal enclaves of South Devon looking for clues to unanswered questions about the great crime author's life and times.

There will be a rare chance for an atmospheric twilight visit of Greenway in mid-September as part of the anniversary celebrations, as well as a special lunch on nearby Burgh Island, where And Then There Were None and Evil Under the Sun were set. A murder mystery will also be staged at the Cary Arms, which enjoys an idyllic, secluded seaside location beneath the cliffs of Babbacombe Bay. This formerly rundown pub has fast become a destination hotel. While retaining its essential pubbiness at ground floor level, the rooms have been exquisitely redone by Peter de Savary. There are cottages too, for those with families (and dogs) in tow.

One of the nicest things to do here - other than eat super-fresh sea bream and other fishy delights on the terrace (try to bag the Captain's Table, perched right over the sea in a gazebo), or get a  divine massage in the one-room spa - is to take the hotel's little boat around the shoreline to Anstey's Cove.  This is the site of another Christie mystery: who was the young man she picnicked here with and became fervently attached to. but refused to meet again as an adult? The young Agatha broke many hearts in and around South Devon, becoming engaged to Reggie Lucy, whom she spent years roller-skating with on Torquay Pier, before losing her heart to a young aviator called Archie Miller at the outbreak of World War I .

Their honeymoon hotel, the Grand, is one of several landmarks along the Christie Mile in Torquay. Though barely 15 minutes' drive from Babbacombe, the beauty of the bay is that it's so shielded there is no sense of a bustling resort just the other side of the cliffs.

Had Archie not gone straight to the front, we might never have enjoyed Agatha's immense literary legacy. Looking for war work, she trained as a chemist at the Torquay Dispensary, and there acquired her interest in poisons. In 1920, her lucrative writing career was launched by a novel in which she described in graphic detail a poisoning death. A new poison-plant garden at Torre Abbey, near Christie's childhood home, is a stop worth making on the Christie Mile; it also houses a charming period home open to visitors.

Nostalgia buffs will want to continue around the sweep of the English Riviera - as Torbay styles itself - to Paignton, where it's possible to ride a few short but scenic miles on the steam train Agatha once rode to London. The train (whose operators are staging their own murder mystery dinner aboard as part of the Christie Festival), halts  beside a heart-stopping view of Dartmouth Harbour, accessible by a short ferry ride from the station. From here it's possible to take a World War II boat up to Greenway, which is a magical way to arrive at the Christie holiday home, where both Agatha and Max died after 40 happy years together.

Do pause, however, in Dartmouth, a delightful shopping town which caters particularly well to foodies. The Michelin-star-winning Carved Angel may be long gone (the New Angel, which came up in its place has lost bravura chef John Burton-Race), but Mitch Tonks has taken over as gourmet king of Dartmouth with his Seahorse restaurant, offering an abundance of fresh fish. Fine fishy fare overlooking the river from a terrace is also available at the Dart Marina Hotel, which turns by night into the kind of trendy chill lounge. Salcombe down the coast, where many a Miss Marple and other Christie tales have been filmed, is another source of fishy gastronomy.  

Here the kitchens of the family-owned Soar Mill Cove and the stylish new South Sands Hotel have been praised for their treatment of fresh catch. Even breakfast is an event at local haunt Captain Morgans, recently taken over by the owners of the Victoria Inn, the town's best pub. But what people like best about this charming resort is an old-worlde seaside village feel and a sense of messing about in boats.

There is good eating too, in the very family-oriented Teignmouth where, just across the bridge in Shaldon, Roux-trained chef Tim Bouget has got foodies' tastebuds tingling at Ode. 

Pretty well anywhere in Devon expect to find the country's best teas, with lashings of local cream to heap atop home-made scones and strawberry preserve. Perhaps this is why Agatha found it so hard to tear herself away from her green, pleasant and almost unbelievably chocolate-box pretty home county.

Getting there

Cary Arms (01803 327110; www.caryarms.co.uk) has double rooms from £150 per night, including an excellent breakfast. More information on Agatha Christie Festival events, at www.visitsouthdevon.co.uk

Don't forget your travel insuranceSave money on airport parkingFly/Drive? Organise your car hireCut the costs of mobile phone calls abroad