It is a critical cliche to call plays disturbing. But Into the Numbers, by the Chinese-American playwright Christopher Chen, genuinely is. It explores the story of Iris Chang, who killed herself in 2004, seven years after the publication of her bestseller The Rape of Nanking. Subtitled The Forgotten Holocaust of World War Two, her book described a massacre in which Japanese soldiers killed 300,000 civilians and raped up to 80,000 girls and women in Nanking, China, during six horrendous weeks in 1937.

Chen uses several means of getting to the heart of the matter. The play begins as a lecture in which Chang sets out the basic facts recounted in her book. A subsequent public interview turns into a form of nightmare in which she is confronted by victims of the atrocity as well as by a survivor, a Christian missionary named Minnie Vautrin, who killed herself on returning to the US. The actor playing the Interviewer morphs into Chang’s husband, Brett, and also her doctor – both of whom seek, in different ways, to rescue her from acute depression.

In 90 minutes, the play raises any number of issues. The most obvious is whether detailed research into genocide inevitably has a destabilising effect. How do you live, Chang asks herself, with the recurring image of babies thrown into the air and bayoneted? Chen suggests, as the Interviewer tries to relate the Nanking massacre to other mass killings, that it is impossible to quantify evil. The play also endorses Chang’s fury at Japanese unwillingness to offer a public apology for the events at Nanking. Yet for all its multifaceted approach, the play never fully explains why Chang, who went on to write another acclaimed book, The Chinese in America (2004), suffered a nervous breakdown at the peak of her success.

Elizabeth Chan gives a remarkable performance, showing how Chang’s initial public assurance gradually disintegrates: one minute she angrily rebuts an evasive Japanese diplomat, the next, she retreats into moody solitude, listening to Ray Charles records. Timothy Knightley switches with seamless skill from the Interviewer to Chang’s concerned husband and bewildered doctor, and Amy Molloy is quietly impressive as the missionary who believes she has witnessed the devil at work. Georgie Staight directs with great clarity a sombrely haunting play that reminds us of past crimes and exposes the high price paid by the individual for uncovering historical injustice.

• At Finborough, London, until 27 January. Box office: 0844-847 1652.