The short answer is no. There is no evidence that the Templars possessed the Holy Grail or indeed any other mystical relic. Recently one of the leading historians of the Templars, Helen Nicholson, summed them up thusly: "As far as the Templars went... [the] evidence we’ve got shows them to be extremely boring Roman Catholics".

There are three common types of Templar myths. Firstly, and perhaps the most common, is that they secretly survived their suppression in 1312. Second, that in the Holy Land they found some priceless Christian relic (often the Grail, sometimes the Shroud of Turin). Third, that they were demon worshippers who practiced secret rituals and magic.

There is no evidence to support the claim that the Templars survived in secret after 1312. A very common story is that the Templar fleet, before the first arrests, escapes from La Rochelle in France and goes into hiding either in Portugal or Scotland (sometimes even the then undiscovered Americas). This story is completely implausible. The Order had no foreknowledge that they would be arrested by the French king and accused of heresy. Only the day before their arrest, the Grandmaster of the Temple had been a pallbearer at the funeral of the king's sister-in-law. The arrests came as a complete surprise. The Order had no time to prepare, let alone spirit all their wealth out of the country via La Rochelle. When the Order was finally suppressed, those brethren that had not died or been executed in the course of the trials were assigned penance and went to live out the rest of their lives at other monasteries. In 1338 the Hospitallers, another military order like the Templars, were still caring for 12 ex-Templars in England. The trial record for the two Templars put on trial in Scotland is incomplete, and so we don't know what punishment they were given, but it is likely they were also transferred to another monastery. The Scotland link is often built on a misconception that the country was a backwater and hence a good place for the Order to hide, not only is this an inaccurate depiction of medieval Scotland, but there is also no evidence of a Templar presence after 1312. Some pseudohistorians have also claimed that former Templars helped Robert the Bruce win Bannockburn. There are no contemporary sources that make this claim and the idea that a king desperate to win papal support in his struggle with England would side with members of an order suppressed for heresy is illogical. In Portugal the Templars were suppressed, but their property was transferred to a new military order, the Knights of Christ. This event has been interpreted by many writers of Templar mythology as the Portuguese king allowing the Templars to continue in Portugal under a different name but, as I understand it, only their property was transferred to the new order, not personnel.

The stories that the Order uncovered some hidden relic in Palestine, whether the Holy Grail or otherwise, are also baseless. The major relics discovered in the East during the crusades, the Holy Lance and the True Cross, were in no way linked with the Templars. The Lance was discovered in Antioch during the First Crusade, several years before the Templars were formed, whilst the True Cross was had resided in a church in Jerusalem long before the crusades even began. Another strike against this claim is that there was little or no value to keeping a relic secret. Relics were massive attractions for pilgrims and could greatly improve the morale of a medieval Christian army. If the Templars had really found the Grail, you can be sure they would have either put it on display in Jerusalem and charged visitors, or have used it to bless their soldiers before battle.

Historians are still divided over just how much of the heresy charges against the Temple were true, but certainly any claims of magic or outright heresy have dismissed. Instead whatever heresy they did commit has been seen by historians as motivated by ignorance. For example, several knights claimed in the trials that the Grandmaster of the Order could grant them absolution, this is backed up by some versions of the Order's regulations. However, the wider Church disagreed and claimed that only ordained priests were able to give absolution.

These conspiracy theories about the Templars stem from a range of sources. Their link to Freemasonry, for example, was actually created by the masons themselves in the eighteenth century in an effort to construct a more elaborate origin for their fraternity. Other stories have arisen more recently. The major modern instigator of Templar mythology is Michael Baigent, Richard Leigh, and Henry Lincoln's The Holy Blood and the Holy Grail (London, 1982). They were the first modern writers to connect the Grail with the Templars (and their ideas influenced Dan Brown's Da Vinci Code so much that they, unsuccessfully, tried to sue him).

A good introduction to the Order, and one that's quite accessible, is Nicholson's The Knights Templar (Sutton, 2001). If you're after a more indepth academic one, then Malcolm Barber's The New Knighthood (Cambridge, 1994), would be a good option. For Templar mythology specifically, see John Walker, ''The Templars are Everywhere': An Examination of the Myths Behind Templar Survival after 1307', in Jochen Burgtorf, Paul Crawford, and Helen Nicholson (eds), The Debate on the Trial of the Templars, 1307-1314 (Farnham, 2010), pp. 347-358. Walker is also working on a book on Templar myths, but no release date has been set yet.

If you're unsure on whether something to do with the Templars (or indeed any aspect of history) is true or just pseudohistory or some sort of conspiracy theory, then a few aspects of the book you can look out for. Firstly, is it written by an academic (i.e. do they have a doctorate or a post at a university)? Is the book being sold as a 'secret', 'hidden' history, or one that claims to tell the 'true' story? If it is written by an academic and cites its sources, then it is generally safe. If it sells itself as being revolutionary or revealing some secret history, then you should be cautious. These aren't hard and fast rules, but can be good guidelines. Another thing to check is to look at reviews of the book in academic journals online.

Sources:

Michael Baigent, Richard Leigh, and Henry Lincoln, The Holy Blood and the Holy Grail (London, 1982)

Malcolm Barber, The New Knighthood (Cambridge, 1994).

Jochen Burgtorf, Paul F. Crawford, Helen Nicholson (eds), The Debate on the Trial of the Templars (1307-1314) (Abingdon, 2010).

Lambert B. Larking and John M. Kemble, The Knights Hospitallers in England: Being the Report of Prior Philip de Thame to the Grand Master Elyan de Villanove, for 1338 (London, 1857).

Helen Nicholson, The Knights Templar (Sutton, 2001).