The 'Endeavour' series is 100% superb all the time; I've seen the whole series (to date) and this is definitely the very best episode of all. 'Quartet' is, in an hour and a half, actually a very deft self-contained Cold War spy thriller that could easily have been issued (with Sean Connery, and added-in action, and a Bond girl) as the James Bond film in the 1968 gap in that franchise.
The story opens with a murder at a clown-giant race with international teams competing in Oxford on live (1968) TV. Endeavour is quick to realise there is more here than meets the eye. The plot continues using all the best Bond tropes - international intrigue, secret spies in HM government, a late-night rendezvous, a code-breaking mystery, a psychopathic industrialist, a clue hidden in a poisonous-fish tank, villains who seem sympathetic victims at first, government's doubt in our super-spy hero, untrustworthy women, and, best of all, an esoteric literature-based riddle that only our hero has the intellect to work out.
On top of all this, it is beautifully produced, with each scene patiently filmed even whilst the plot runs at breakneck pace. As ever for this series the historical details are spot-on and charmingly nostalgic. And there is the start of the secondary subplot that will become the principal plot of Series 6.