Agent 6 is the third in Tom Rob Smith's trilogy about a former MGB agent, Leo Demidov. It opens with a flashback to the younger Leo in 1950, a committed secret policeman of 27 who has just discovered the of a young artist. Polina Peshkova has committed no crime beyond wanting to keep for herself some private space, but like millions of others in the Soviet Union, she is sucked into what Evgenia Ginzburg described as "the whirlwind" of arrest, interrogation and punishment. Her mistake has been to leave a single sooty fingerprint on her writing desk, and this is enough for Leo to deduce that the diary may well be hidden in the chimney. The consequences of the investigation begin a journey that will strip from Leo the satisfactions of career, status and certainty.

In the first of the trilogy, the Booker-longlisted Child 44, Leo tracked a serial murderer and in doing so unravelled the fabric of his own past. In the second novel, The Secret Speech, he was faced with the consequences of his brutal career, after Khrushchev's 1956 "secret speech" to an unofficial, closed session of the Twentieth Party Congress. Khrushchev's denunciation of Stalin was explosive, implicating leadership, secret police and judiciary in systematic intimidation, false accusation, torture and execution of "individuals who had not committed any crimes against the party and the Soviet government". The fictional Leo Demidov, like so many who had believed they acted with impunity, discovered that this was not the case. If the dead could not rise, others might avenge them.

In Agent 6, the action moves between the Soviet Union, New York and Afghanistan. By 1965, Leo, his wife Raisa and their two adopted teenage daughters are living in a cramped apartment that offers the standard living space of the typical Soviet citizen. Privilege has gone but, very slowly, trust has grown between the four of them. Leo's MGB past cannot be buried but it seems to be quiescent.

Once again, a diary is the lever that exposes Leo's weakness. One day he detects that his daughter Elena is keeping a secret diary, removes it from its hiding place and is about to read it when he stops himself. This is not an investigation, and Elena is not a suspect. Leo will not return to his former role.

However, the decision not to read the diary turns out to have terrible repercussions. Had Leo read it, he would never have permitted Elena or her sister Zoya to travel to the United States on a school choir tour organised by his wife. The vision is one of Soviet teenagers and American high-school youngsters joining hands in a symbolic sharing of music. The reality is a nightmare of blood and betrayal.

The plot of Agent 6 is highly complicated, and its twists and turns strain credulity, especially in the final third of the book, where the action switches to a not-very-convincing Afghanistan. Smith seems to believe that only a busy succession of scene changes and increasingly improbable action will keep his readers' attention, and rarely lingers to explore a situation, reaction or relationship as he did in Child 44.

The emotional and moral heart of all three books concerns an understanding of the flawed, even criminal self. Leo neither loves nor admires himself, once his early self-regard as a highly effective, handsome young MGB officer is ruptured by events. He learns very slowly that he too is down in the mire, and must build resistance within himself to the overwhelming pressure of Soviet society, if he is not to succumb to it. However, the writing in Agent 6 cannot sustain tragedy, alienation or despair.

The best thrillers combine narrative tension, first-rate plotting and enough psychological insight to satisfy the human hunger for identification. Child 44 made it clear that Smith can do all this. In addition, he takes a highly visual, filmic approach, using flashback, establishing shots and close-up with panache. He knows how to make us focus on Leo as he crosses and re-crosses the canvas of time. The blend is less successful in Agent 6, and there's a disappointing loss of momentum. The more strenuously action-packed the story becomes, the less engrossing it is. It's as if the author believed that the book should not gamble with moments of inward intensity, although it's these moments that make the hairs prickle on the back of the neck. Tom Rob Smith is not at the top of his game in Agent 6, but he's a thriller writer with huge potential. If he takes more risks he may pull off something even better than Child 44.

Helen Dunmore's The Betrayal is published by Penguin.