What does it mean it be good? That's the question at the heart of Canberra playwright Tom Davis's new work, The Faithful Servant, which will have its premiere at The Street Theatre in September.

Davis, whose play The Chain Bridge was on at The Street last year, says The Faithful Servant , set in Australia and Mozambique, centres on three main characters, each with his or her own idea of what being good means.

Raymond (played by P.J. Williams) is a doctor who back in the 1960s set up a hospital and clinic in Mozambique. He and his wife adopted a daughter, Caroline (Tariro Mavondo), and eventually brought her back to Australia where she has grown up. The third character, Coetano (Dorian Nkono) is a Mozambique local who has worked with Raymond and is still trying to help his country.

"It's set at the end of Raymond's life, roughly the present. He feels he's failed in all he's achieved. He wants his daughter to go over and take over the reins but Coetano wants to her to go over for different reasons: his vision is different."

Raymond went over to Africa, Davis says as an idealistic young Methodist with a sense of adventure and "a slightly old-fashioned view", that of someone bringing his knowledge of Western medicine to a poor country to deliver that knowledge for its benefit – a somewhat patronising, if not racist, viewpoint but a well-intentioned one.

"Then something happened: he saw the reality of the fact of what real poverty means. It changed him and changed his idea of what he did."

But what he thought he could do in the 1960s and what he feels about himself and what he has accomplished 50 years later are very different.

"The world is not as easy to change as he thought it was."

For some people this realisation – and the related revelations that they can't save everybody and that their intervention might sometimes make things worse rather than better – comes as a challenge to their conscience and intentions or even their faith.

On the other side, Coetano, Davis says, "is trying to do good for his country any way he can ... He will do whatever it takes."

If that involves, for example, co-opting religion in a somewhat cynical manner to try to attain some material benefits for people, then that's what he will do. He isn't above adopting the "prosperity" techniques of some churches: "If you pray hard enough and do certain types of things then good things will happen – it's religion as a tool rather than a deeply held belief."

Although she is African by birth, Caroline has grown up Australian and doesn't want to uproot her life to go back to Mozambique, Davis says.

"Her vision of doing good is about family, bringing good to the family and the community in which you live: you can do good in Australia, you don't have to travel across the Indian Ocean.

"The play is asking, why is it so difficult to be unambiguously good? The play is trying to say, it's complicated – having empathy for other people, people of a place so different from yours, some not caring, a clash of world views, a clash of cultures, and different interpretations of what it means to be good."

The play's title refers to the biblical parable of the faithful servant and Davis says all three characters are "faithful servants – to each other and to their own ideas of what goodness is".

They might discover, he says, that they have more in common than they realise.

Davis has worked in the field in foreign aid for many years in the Pacific and south-east Asia and has seen the difficulties as well as the benefits in helping communities. although he hasn't worked in Africa, relying on research and interviews for that particular material.

"One of the reasons I've chosen Mozambique is that I wanted a place that was very, very foreign to most Australians. Australia has a reasonable connection to the Pacific and south-east Asia, we sort of know these places a little bit, but a lot of Africa is unknown, it is still 'the Other'."