B-File, based on text by Deborah Levy, directed by Paulo Castro. Co-written and performed by Jo Stone, Karen Lawrence, Paolo Dos Santos, Paulo Castro, Silvia Pinto Coelho and Madeleine Lawrence. Produced by Tanzfabrik-Berlin, Teatro Vila Real-Portugal and Casa Das Artes Familcao-Portugal, La Mama @ the Courthouse until April 14.

Airports accrue a peculiarly modern form of paranoia. Not only are they places where you wait, entering that "room of lost steps" where time becomes another dimension altogether; they are spaces of transition, where you are neither in one place nor another. You have no context in which to expand your identity, no way of demonstrating anything about yourself except through those crucial documents - passports, visas, tickets - that establish your claim to be a human being, a citizen, a whole person with rights and a proper name.

When I'm travelling, I always have a moment of anxiety if, for any reason, my passport leaves my humid grasp. I remember once, travelling by train from Switzerland to Italy, total panic rising in my throat when the Italian conductor brusquely took my passport overnight (to save, I found out later, the Italian immigration officials the task of inspecting papers from carriage to carriage). It doesn't take much imagination to extrapolate that anxiety to the actual situation of a person with no papers, no official identity, no legitimate means of moving through the rituals of citizenship. Which is the position, for example, of a refugee, or a prisoner in Guantanamo.

It's a condition of legal non-being that the philosopher Giorgio Agamben has named "naked life": a state of legal exception in which the full power of the state might be exercised against an individual who has been stripped of any rights of citizenry. When we say people are "dehumanised" by, for example, being put in concentration camps, we actually mean that they have been delegitimised: they no longer have any rights as citizens, and literally anything might be done to them.

The fear that our citizenship may be denied lies behind the anxiety we feel at airports, when our identity, which must be demonstrated at every turn, feels particularly vulnerable. And B-File - now on in its second week at La Mama, so hurry if you wish to see it - is a dark fantasia that begins precisely from this state of anxiety.

A fascinating and classy dance/theatre piece adapted from a text by Deborah Levy, B-File is a German-Portuguese co-production spoken in six languages. The show opens with three women sitting in a row, waiting, all of them with the bored, resigned expressions of travellers. Two policemen enter, and after making an obscene gesture, so swiftly that you almost don't see it, beckon one of the women to stand on a particular spot in front of them, and demand to see her passport. The other passengers watch, with the palpable relief that it isn't them.

It seems that she doesn't speak English; she is Japanese. In a sequence that is like a nightmare, she can't find her passport. She searches with increasing panic the bag around her waist, all her pockets; it isn't there. She tries to open her handbag, but the zip is stuck. She pulls, like a magician pulling scarves out of a hat, brightly coloured items of clothing out of the bag's narrow opening, staggers around the stage in a strange, dislocated dance and finally lies down as if she is going to sleep. When she's roused from her sleep, she escapes into the toilets.

Finally she does begin to speak. No, she doesn't have a passport. They policemen want to know what she says to her lover when she makes love. She tells them. She will fuck one of the policeman, doesn't he want to fuck her? Is he a poof? (What, the German policeman asks the other, is a poof?) She is threatening, she is disobedient, she runs away and disappears. She is a fantasy.

The policemen then turn their attention to the other travellers. The same game is played, but this time it is nastier. The second woman is a Portuguese, a dancer, and this permits a few jokes about contemporary dance, and links the policing of aesthetic boundaries to the question of state power. The policemen empty her bag, throw her belongings on the floor. She is obedient, polite, attempting to appease her interrogators by being as invisible as possible, but that doesn't help her. Like everyone else, she is forced to prove who she is, to perform her identity. And this demand is overlaid with an even more sinister sense of sexual sadism.

One of the male policemen leaves, and a woman takes his place. Suddenly the gendered game that has been played becomes more complex; the woman policeman is more brutal than the man. The next interrogation, of a middle aged Australian woman who begins by assuming her legitimacy, is nastier still in a way that reminds me of Pinter, especially in the nervous explosions of laughter the performance generates: it is blackly funny, but you can't feel comfortable. The threat is palpable, but it is impossible to know what is being demanded. The only thing that is visible is the exercise of power and the distress of those at its mercy.

The different languages spoken are partly responsible for the anxiety this piece generates: you are constantly catching up with what is going on, guessing at first and comprehending later, as it is translated into English. It's heightened by explosive segues into dance, and the two or three times there is a sound effect - a sudden very loud noise that obliterates all other sound - that is disorientating, as if, paradoxically, you are suddenly deaf.

Paulo Castro's direction has a spare simplicity that focuses all your attention on the action. There is something in the final sequence that has a touch of moral closure, a sense that it is a little too pat, although it has a certain logical compulsion. It is perhaps a little comforting to think that the present state of paranoid surveillance will self-destruct. But B-File is, all the same, an unsettling and powerful contemplation about contemporary authority.